> Think of it like an offsite backup

I still can't think of what sequence of disasters would lead to Earth being less habitable than Mars. Earth still supported life the day after the meteor hit. To bring Earth down to Mars level something would have to remove all the atmosphere, remove all the mineral resources, remove the oceans, and screw up gravity and the diurnal cycle.

Then think of all this talk about Mars or O'Neill cylinders or whatnot as a step toward interstellar travel, and truly off-site backups.

Good point. I see Mars as a lateral move in that quest. More likely that we'll be post-human before we're an interstellar species.