I don't really understand this sentiment, on either side. If we're able to set up a self-sustaining settlement on Mars, then we're going to be able to survive

I don't really understand this sentiment, on either side. If we're able to set up a self-sustaining settlement on Mars, then we're going to be able to survive on Earth, no matter how badly we mess it up. Just build a bubble and pretend you're on Mars. Short of total thermonuclear war, it's hard to imagine any non-survivable world where we have that technology. On the other hand, _someone_ building a backup colony on Mars because _someone else_ decides to get in a nuclear war doesn't sound unreasonable. Plus, there are world ending scenarios outside our control. Meteors, volcanic activity, etc.