I'd buy into Musk's Mars bullshit if he built a sealed self-contained colony in New Mexico. We have experience with two long term space habitats, Mir and the ISS. Despite all of the engineering know how that built both shit breaks on them all the time and needs to be repaired. They also need regular resupply because they're nowhere near self sufficient. They also need a Soyuz capsule docked at all times in case they need to evacuate.

The Starship is an iterative design with seventy years of engineering knowledge behind it. Keeping humans alive outside the Van Allen belts for a long duration and then on a completely inhospitable planet has very little engineering knowledge behind it.

Also to suggest a Mars colony is a contingency plan is ludicrous. In order for a Mars colony to be a human civilization contingency plan it would not only need to be self sustaining but self perpetuating. Otherwise a life ending event on Earth would just give the Mars colonists a few months to come to terms with their mortality. Then no kore humans.

There is nowhere in the solar system that can support humans in the way Earth can. In fact everywhere else is so hostile we need to take significant amounts of Earth with us just to not die immediately.

If you want to live in underground colonies have the Boring company build you tunnels here on Earth. At least when you need parts for the GECK you can go above ground with just a gas mask and rugged clothes instead of a bulky space suit.