In principle, it would be neat to have a backup planet in case something happens to this one. But in practice, everywhere apart from Terra is the absolute worst. Luna & Mars suck less than like… Venus, but that's not a great bar.

Terraforming begins at home. If you want to terraform Mars, how about… fixing this planet where billions of people still live, and will for the foreseeable future? We don't need to forget space, but there's so much more work to do to settle Mars, and most of it doesn't look at all like settling Mars.