Another way that I like to describe this is that almost nothing could make the earth less hospitable than the moon or mars already are. Asteroid strike? Mars' thin atmosphere is even more vulnerable. Climate change? The moon swings between -170C and 119 C each month. Nuclear war? Try living under constant cosmic radiation in a pressurized habitat that could be completely destroyed by a few small conventional explosives instead of a nuclear bomb.

If we had the ability to make a self sustaining colony on mars we'd be able to restore earth's biosphere or make self sustaining arcologies or bunkers.