It's absurd to think that any rational person thinks that finding a suitable or terraforming a planet is a viable alternative to preserving the one we have. At the most living on Mars could be seen like a bomb-shelter for the privileged few, but even then not very soon so they'd also need life extension to make that work. At the same time, there's no reason not to continue to explore and expand. That's what tribes did when they discovered they weren't alone, and countries, and continents, etc.