> Mars has vast mineral resources.

So does Earth.

The downsides of Mars:

• Colder than Antartica • Drier than Sahara • Lower air pressure than top of Mount Everest • Soil perchlorate concentrations on par with a Superfund cleanup site • Atmosphere contains negligible oxygen, nitrogen; you need the former, plants need the latter, you need the plants • No ozone layer (not that you're ever going to be outside, what with the air being unbreathable in two distinct ways) • No magnetosphere (so CMEs are dangerous rather than being pretty light shows) • We don't even know yet if the lower gravity is important

I say this as one of the people who finds Mars inspiring, and would even consider being a Martian colonist: The technical capacity to make Earth as uninhabitable as Mars already is, would also threaten Mars.

Nukes? Mars colonies are much more vulnerable. Engineered viruses? If they existed, private companies could already ship them to Mars — etc.

If your goal is a backup for repopulating after a disaster, we can already do hermetically sealed metal boxes isolated from the outside world for long durations at a time, where at least one such box of people is always secreted away in an unknown location at any given moment: nuclear submarines.