Earlier quoted context omitted.
But what catastrophes could render the Earth so much more uninhabitable than Mars everyday?
You don't design safety systems for every day, you design them for the worst day. For example, on earth there are nuclear weapons, so anything designed to last has to be able to survive a direct hit by a nuclear weapon. Even though a post-nuclear earth would still be nicer than mars, it's easier to survive on mars for centuries than survive that fraction of a second of nuclear fire. And it's not just nuclear - if you…
I disagree. To the extent that you can stockpile supplies to survive on mars through the nuclear event, moreso it is easier to stockpile supplies to survive in underground bunkers.
> And it's not just nuclear - if you are going to have a single point of failure, it has to survive every possible thing - it needs to be nuke proof, germ proof, chemical proof, earthquake proof... Redundancy is much cheaper.
I agree, and it's easier to design separate Earth shelters to withstand different catastrophe scenarios than elsewhere.
> Presumably the martian back-ups would return to earth as soon as the danger of the calamity had passed, so you still get all the benefits of Earth's habitability afterwards. Only the pre-catastrophe society needs to build and support a mars base.
Likewise for Earth-based shelters.
And let's not forget that there are far more failure scenarios and it's harder to recover from them for a shelter based in Mars than for one on Earth.