If you put it on earth, it has to survive whatever renders the earth uninhabitable. Given the very large number of potential bad things that could happen, something that can confidently survive all of them would be very difficult to make.
If it's on another world, it is obviously already in a pretty inhospitable environment that is expensive to access, but that environment is unlikely to get catastrophically worse at the same time as the earth, and the costs of building it can be reasonably estimated.
If you take out a 500k life insurance policy, if you take into account the time value of money it will on average cost you more over the course of your life than the policy actually pays out. You are technically losing money by having the insurance. However in exchange you put a known upper bound on the amount that you lose as opposed to the person who makes the rational choice to forego the policy and assumes unlimited risk. Likewise beefing up the one basket we keep all our eggs in is almost certainly more economical than getting another basket, but it is inevitably still a single point of failure.
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 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.
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.
> it's easier to survive on mars for centuries than survive that fraction of a second of nuclear fire.
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.
If it's on another world, it is obviously already in a pretty inhospitable environment that is expensive to access, but that environment is unlikely to get catastrophically worse at the same time as the earth, and the costs of building it can be reasonably estimated.
If you take out a 500k life insurance policy, if you take into account the time value of money it will on average cost you more over the course of your life than the policy actually pays out. You are technically losing money by having the insurance. However in exchange you put a known upper bound on the amount that you lose as opposed to the person who makes the rational choice to forego the policy and assumes unlimited risk. Likewise beefing up the one basket we keep all our eggs in is almost certainly more economical than getting another basket, but it is inevitably still a single point of failure.