> This argument would have a lot more merit if Earth and Mars were similar, but they're utterly not.

So where's the better choice for redundancy?

> The difficulty, expense, and risk of setting up life on Mars absolutely pales in comparison to cleaning-up life on Earth.

First, that's irrelevant, as a sustainable colony on Mars provides something that you don't get no matter how much you clean up Earth. It can be pristine, it still doesn't get you redundancy.

Second, I don't think it's true that the price is so far in that direction. Cleaning up Earth is monumentally expensive as well, depending on what you even mean by that.

> Suppose there were a Mars colony. How many years before that colony could go on indefinitely if it didn't have support from Earth?

Sustainable in that it can produce the materials needed for modern living? I very long time, I imagine. Sustainable in that it provides enough food and water that a group of people could survive without having to worry about modern conveniences? Much less, and that's always got to be the first step, so that's what we go for.

Even if people on a mars colony ended up living a more simple existence in the case of the loss of Earth, that's still a chance for them to advance and continue that wouldn't exist with the world ending disaster on Earth.