An easier job would be maintaing the habitability of the near-perfect planet we already have x) but it seems we can't even to that! Seems silly when SV tech bros say terraforming other planets will be what "saves humanity" or some such vs.
What it gives us is a backup plan for humanity. There are many reasons the Earth could become uninhabitable, hardly all of them caused by man. For example, an asteroid hit.
0. climate change has already entered an irreversible spiral
1. decades from now, the irreversible spiral having deepened and accelerated, civilization as we know it has been destroyed, leaving no authorities to do anything about climate change
2. decades to centuries from now, climate change makes earth mostly uninhabitable
3. hundreds or thousands of years in the future, if they weren't all dead, humans perhaps would have been able to maintain a self-sustaining colony of all for humanity another planet
there's not a realistic path for humanity to survive an earth-destroying asteroid, and there never will be
