> If an asteroid is going to wipe out Earth, we can't evacuate 8 billion people, or even 8 million people. but if we had a few generations of colonists in domes on Mars, or floating in the clouds on Venus, or somewhere else, the species itself would hopefully survive.

The problem with this reasoning is that even in the case of a dinosaur-busting asteroid impact earth will be still much more liveable than mars.

the proof being that earth got a dinosaur-busting asteroid impact few millions years ago and that life continued there but didn't appear on mars