I don't see a likely future where space colonization as fleeing a ruined earth, but one pan-solar civilization _includes_ a vital and flourishing earth.

Briefly: we can use the resources of the solar system to make life here better. Trading with Earth will make life possible and profitable Out There.

Which, I know, is so-very 70s High Fronter of me. But, man, wealth makes anything possible and there is a whole lot of wealth in the solar system, just waiting to be properly utilized.