Earlier quoted context omitted.
Why do we need to fly 3 months to some inhospitable planet that is in worse condition than Earth would be (even after nuclear wars and global warming) to make progress? I suppose there's an idea we could apply everything done on Mars to Earth, but like... seriously, what we just took all the prep for mars and just put it in the middle of the Empty Quarter, the Saraha, Antarctica? People barely want to live in the arc…
The basic idea is to not have all your eggs in one basket. We are one asteroid strike away from losing all human life on earth. Having 2 habited planets spreads the risk. Obviously having those planets in different solar systems would be even better, but we don’t have the technology for that, yet.
We're one asteroid strike from losing all human life in the solar system even if you had a bunch of people living on Mars. It would take a very long time to get even a single Mars colony self sufficient. Without resources of Earth supporting the colony it would die out the first time the air conditioner broke.
Putting people on Mars isn't putting our eggs in a second basket. It's putting an egg in a wet napkin on a mouse trap positioned under a precariously balanced boulder.