All existing human cultures are deeply founded on a pre-Newtonian and pre-Copernican world view, in which the Earth is "special" as the stage for life and human activity, amd the rest of the Universe is merely a background. This prejudice is reinforced by our ignorance: the Earth is, in fact, the only part of the Universe about which we possess much detailed knowledege, so it is easy for us to imagine that it is the only place of any importance.
If we bother to seriously think about it, this attitude is obviously absurd, but it is an easy opinion to return to, and we return to it at every chance, finding new excuses in each case. The implicit excuse of the Mundane SF movement is that we have serious ecological problems on the Earth and that thinking about colonizing other places "distracts" us from solving them.
The first fallacy, of course, is a variant of that of the Excluded Middle. "We" (Mankind) are not a unitary entity which can only think about colonizing other worlds or problems on Earth, one or the other. Indeed even "we" in the sense of each separate nation are not so bound: it is quite reasonable to envision some people in each country working on Earth-based ecological problems while others work on colonizing other worlds.
A deeper error lies in the assumption that activities "in space" and activities "on Earth" must compete in the long run for resources: that effort we put "into space" is somehow lost to "the Earth." The reason why this is an error is that progress is synergistic and often serendipitous: advances made in one field often are prerequisites or triggers to advances made in another field, as explored in the classic series Connections http://en.wikipedia.org/wiki/Connection
We've already seen this, more than once, in regards to this very issue. When we were first able to see the whole Earth from space, we realized that she was a planet and a unitary thing as we never had before, which gave impetus to the environmental movement. And the development of Landsats made it possible for us to monitor global processes on a global scale, without restrictions created by the accident of accessibility to ground and air expeditions.
We can expect further advances in the future. As the cost of Earth-to-orbit and orbit-to-Earth travel drops, Earth will become economically connected to extraterrestrial resources -- first to terrestrially rare ones such as tri-helium and certain uncommon metals, eventually to others as the cost drops even further. Mining and industry will increasingly move off-Earth, enabling Earth to become a garden planet.
But the truth is starker than this, because I've just described it from Earth's perspective. Though there will long be a practical, and still longer be a sentimental human tie to the motherworld, in the truly long run, Earth is only one planet among literally trillions in our Galaxy, and quintillions to sextillions in the Universe as a whole.
If Man is successful as a sapient species, we will expand out into the Solar System, the Galaxy, and the Universe beyond, and one day Earth will be at best only a distant memory, an echo of long ago and far away. And if Man is not successful as a sapient species, then we will perish, and one or more of the unknown number of other sapient species in the Universe will succeed in our place, and to then Earth will be at best an archaeological dig site, possibly not even that, if they don't discover the planet before the Sun expands into her red giant phase and whiffs her into vapor.
This is the true root of the Mundane SF movement: not just Fear of Boundlessness, as I argued before (http://fantasticworlds-jordan179.blogs
To this fear I can offer two counters and a hope. The counters are that it doesn't matter -- reality is whether we choose to pay attention to it or not, and more trivially that places beyond the Earth exist and will be expanded into by other human cultures and subcultures, even if any particular human culture (yes, even America!) chooses to squeeze her eyes firmly shut and chant "you're not there, you're not there!"
The hope is simple.
The Universe is out there. We may be worthy of it. And if we are sufficiently worthy, then Man -- rather than being insignificant, as the insignificance of our birthworld might make us imagine -- might be one of the most significant productions of the Cosmos, as the entity destined to become at least part of the great stream of sapient life which will organize the Cosmos and bring it consciousness. Earth in this view would merely be like a single fertilized human egg, a tiny insignificant thing which can produce the most momentous consequences in the form of a new human being, if it succeeds.
A baby can't succeed if it spends its whole life in the womb. And Man can't succeed if we spend our whole life as a species on Just One Earth.
There's a whole big Universe out there. Let's grow into it.