A post apocalyptic earth after every nation on earth detonated all their nukes on each others population centers or a post dinosaur killer event would both leave the earth substantially more survivable than Mars at present. The world faces unlikely total destruction from space. More likely species extinction, and inevitable economic failure.

In the foreseeable future of the coming decades even inevitable economic failure is likely to doom any colony we establish whereas such colonies are not going to be enough to save the species from any species extinction scenario.

In the near term space exploration is about developing our potential not preserving it from destruction. Indeed this argument is both more palatable and potentially effective than doom and gloom.