That whole romance with the idea humans need to go to space to survive... Sigh. Earth is, and will always be, the most human-friendly planet in our reach. Mars, and any other planet in our proximity, is DEAD. You have to live in airtight bunkers, go out in spacesuits. Even if our planet was hit by a catastrophe like a supervolcano, Earth would still be a friendlier environment than that. If you can build autark colonies on Mars and Venus, you can build autark colonies on a catastrophy Earth.
Then that talk about terraforming. We can't even purposely terraform our own planet.