I have often heard the complaint, addressed to space opera from more serious sorts of science fiction, that space opera is anachronistic. Star-spanning empires? Future religions? Feudal interstellar intrigue? Surely those are the features of the past, not the future. Mankind went through all that in the Roman Empire, rise of Christianity and birth of the West respectively.
I. The Assumption of Succession of Economic and Ideological Systems
The underlying assumption of this argument, though not all who make it are fully aware of its basis, lies in Marxist economic history. According to Marxism, each age is dominated by a "means of production" which results in an "ideological superstructure" which reconciles men to the means. The "means of production" of Classical Antiquity was chattel slavery; that of the Middle Ages agricultural serfdom, that of the Early Modern world (*) commercial capitalism, and that of the Late Modern world industrial capitalism. The Marxists, of course, expected industrial capitalism to be succeeded by a state socialism which, as the state "withered away," would become "full" or "true" socialism (which we might call today "anarcho-socialism.")
And indeed, if we leave out the part that hasn't happened yet (and is very unlikely to ever happen), this is pretty much the pattern we have seen in the history of the Western world from around 500 BC to the present day. We even see a long-term trend toward state socialism, of an authoritarian to totalitarian variety.
By this logic, how could we ever have interstellar "empires," of the Roman sort? The Imperial system was created to stabilize and justify a system supported by slavery, where the importance of stabilization was to prevent falling into slavery being too easy and thus interfering with personal economic planning. Christianity was adopted first to stabilize the Empire, and then the feudal states that the barbarian kingdoms formed in order to raise armies, within the limitations of a pre-industrial agricultural serfdom. Why would either messianic religions or feudal magnates appear, absent those reasons?
But there is another way to look at history, and in this view, a recurrence of roles appears exceedingly likely.
(*) I am using here the scheme of Perry Anderson, probably the best of all the Marxist economic historians. He correctly saw that Marx and Engels conflated the Medieval and Early Modern worlds as "feudal," when in reality Western European "feudalism" had pretty much died by the late 15th century, and the national monarchies that replaced it were something very different: "absolutism," to use his term.
II. Historical Cycles
The concept of cyclicism in history is very old (it dates back at least to Aristotle), but requires some explanation. The basic idea is that human cultures tend to follow a natural pattern of change that eventually returns to its starting point, in terms of economics and politics. The proviso is very important, because technology is not cyclic: technological change, as far as we can tell from the 10,000 or so years that we've enjoyed rapid progress, is progressive. Technology has a "ratchet effect" and it is rare for a technology to be forgotten unless it is replaced by a more effective one.
The Ancient concept of cyclicism did not have this progressivism, mainly because their technology was advancing too slowly for most individuals to perceive in the scale of their own lives, or even known familial histories. Think of the Ancient cyclicism as a potter's wheel, turning but going nowhere; by contrast, Modern cyclical theories view the wheel as mounted on a vehicle progressing along a road. The wheel spins, and winds up further along the road with each turning.
The best-known Modern cyclical theories are those of Spengler and Toynbee. Spengler noted great similarities in the histories of different civilizations when he put their timelines side by side; for instance, messianic prophets and great conquerors tended to appear at roughly the same stages of development for each culture. A particularly useful site outlining this is found at Spengler's Future (http://www.johnreilly.info/title.htm)
Spengler noted that the West seemed to have reached its height of cultural productivity, and that while we might have a long future ahead of us it would in many ways be downhill from here (the early 20th century). This is why his most famous book is entitled The Decline of the West). (http://en.wikipedia.org/wiki/The_Decli
Toynbee attempted something even more ambitious. He catalogued "civilizations" as entities, and showed how they were arranged in "generations," with civilizations not only rising and falling but giving birth to successors, sometimes by a phoenix-like resurrection, sometimes by an almost sexual combination (for instance, he saw the Greco-Roman Classical civilization as having combined with the Western-Semitic one to produce Eastern Christendom and with the Western-Semitic and Germanic ones to produce Western Christendom).
In the view of both authors, history does not repeat precisely, but it does fall into patterns -- patterns which emerge from basic human nature as expressed through culture, economics and politics. In consequence, history does repeat itself roughly.
Toynbee saw the life cycle of a civilization as having four stages, which he termed "Spring," "Summer," "Autumn" and "Winter" (on Classical-Chinese historical models).
In the Spring stage, a civilization is born (usually from the ashes of and/or combination of predecessors). Its basic culture forms -- during this period, it is very susceptible to foreign influences. Wars are almost continual, but usually small in size. For the West, this happened during the Middle Ages. This is generally an extremely chaotic and violent era, often remembered as a "Heroic Age."
In the Summer stage, a civilization grows and explores its possibilities. During this stage it often expands greatly beyond its original homelands. Economic growth is rapid, and there is considerable cultural growth as well, though the civilization is no longer as plastic as it was in Spring. Wars are less frequent but larger in scope. For the West, this was the Modern era from the Renaissance roughly to 1914.
In the Autumn stage, a civilization consolidates. Economic expansion is slower but steadier, and geographical expansion also slows, with an emphasis on fuller exploitation of existing territory. Wars are now infrequent but very large when they occur. During this period the number of Powers is reduced; by the end, one Universal State will emerge which will rule all or nearly all of the territory controlled by the civilization. The West entered the Autumn stage around 1914, and is probably still in it today.
In the Winter stage, a civilization stagnates and dies. The Universal State, once flexible and relatively free, hardens and grows totalitarian. Economic expansion slows, eventually becoming contraction. Geographical growth also slows and then reverses; if any colonies are founded they will not be held. Wars are now either civil (within the Universal State) or are inflicted on the Universal State by more vigorous rivals; in the end the Universal State will go down before the blows of "the barbarians."
From the ashes the phoenix rises again. The barbarians who bring down the Universal State imbibe its culture in the process, and the Winter of the earlier civilization becomes the Spring of the successor civilization. And the wheel has turned, and goes on turning ...
The West has not yet entered the Winter stage, but it is plainly approaching. The likeliest Universal State for us will be America, though a late upset is not beyond the bounds of possibility. IMO (not Toynbee's, as he believed interplanetary colonization to be impossible) the West will colonize other worlds, but lose control of them as the demands of the increasingly-totalitarian Universal State of America grow insupportable, and these other worlds may well be the breeding grounds for the "barbarians" who will bring us down in the end.
The most famous case of a civilization which went through all those stages is that of Greco-Roman Classical culture. Their Springtime came after the fall of Troy; their Summer began with the Persian Wars and continued into the time of Alexander; their Autumn saw the rise of the Hellenistic Successor States (of which Rome was one by choice); and the Roman Republic became their Universal State. The Republic became an Empire, and their Winter saw Rome, which had conquered the Classical world, ossify and slowly lose control of the West to the Germanic barbarians ... who then spawned our Western Civilization from the ashes of the Western Roman Empire.
The interesting thing here is that there are traces of an earlier civilization, the Minoan-Mycenaean culture from whose ashes the Classical Greco-Roman culture rose. Their Springtime is lost to prehistory, but there are archaeological traces of a Summer in which the Minoans colonized the Aegean basin, and mythological traces of an Autumn in which great kingdoms arose both on Crete and on the mainlands, and in which the Mycenaean monarchy became the Universal State by defeating rivals including Minoan Crete and Ancient Troy, only to finally succumb to a Winter in which it perished in the coming of the Sea Peoples, including the Dorians.
III. Applicability to Space Opera
Now, imagine yourself a person living in (say) the Roman Republic of the 2nd century BCE.
Suppose that a poet writes what we might call a "hypothetical epic," in which a hero of the future has adventures like those of Odysseus, trying to return home after an apocalyptic battle but being constantly taken off course by ill-fortune and hostile tribes (we'll leave out the monsters). By virtue of his heroic cunning and fortitude, he triumphs and returns home to his petty kingdom.
Call this a "wine-dark sea opera," by analogy with "space opera." Is it reasonable?
"No!" you would protest, if you are a well-educated Roman. "That sort of thing happened in the Heroic Age, a thousand years ago (*), but now we live in a civilized age in which the Mediterranean has been mapped and strong states exist almost everywhere. Surely in the future these states will grow, until bandit kingdoms and pirates have been eradicated from Our Sea."
And you would be right. In the very next century, Pompey put down the pirates, and the Roman Republic extended its sway over everything West of the Rhine and south of the Dacians, to the Sahara.
And you would be wrong. Five centuries after his own time, the barbarians would flood over the frontiers, and bandit kingdoms and pirates would dominate almost everywhere. The wheel would have turned back to the end of Winter, and the Springtime of the Western Civilization would be its "Heroic Age."
(*) While Greco-Roman Classical culture did not have "archaeologists" in the modern sense, they did have traditional history reaching to before their written records. Because this included genealogies, they could make guesses about when things happened, and these guesses turned out in most cases to be roughly right -- the Classical Greeks and Romans were correct as to the approximate periods of the Fall of Troy and the Foundation of Rome, for instance.
The same applies to us.
Yes, the West has for the most part put aside monarchy (as America did in CE 1776, and as Rome did in 510 BCE). But that doesn't mean that our future doesn't hold Empire (as Rome's did in 33 BCE), or a return to feudalism (as Rome's did in the 6th century CE) or even absolute monarchies.
Will they be the same as what we knew in the Middle Ages or Early Modern Era? No, especially not technologically. The "serfs" of some future Solar Civilization won't be tilling the soil with oxen -- perhaps they'll be grubbing the lithospheres with swarms of nanobots, or tending vast orbital solar power collectors. The "kings" of that future era won't sit on thrones in crowded courts -- maybe they'll inhabit vast computer networks, or occupy powerful space dreadnoughts as if they were their own bodies.
But the flavor of the culture and politics will be similar. And that's all the similarity you need for good space opera science fiction.
Which is as serious as any other projection of an inherently-unpredictable future.