Mars is already uninhabitable, so what would we gain from moving to Mars? The Earth is not going to become as bad as Mars with any possible disaster smaller than a planetary collision.
Also, a Mars colony that could survive indefinitely if the Earth is wiped out is almost certainly impossible, given the extreme conditions on Mars' surface. We don't even know if there is enough water ice on Mars today to sustain a human city, nevermind the huge network of cities that would be required to produce all high tech products on Mars. Also, with no fossil fuels, it's very unlikely that you could launch rockets from a Mars colony entirely cut off from the Earth.
Then, you'll also be missing nitrogen, and probably a lot of other nutrients. Oxygen will be in short supply, as will Helium. And remember, every time you leak water or other gases into the atmosphere, you've likely lost them forever, since there is no closed cycle of water on Mars, and solar winds are constantly stripping away Mars' atmosphere.
Also, the cost of building a self sustaining colony on Mars is well above the GDP of the world many times over. It would be many times easier to establish a second global trade network on Earth, completely isolated from any existing trade hub. Imagine building chip fabs and growing enough crops for food and bioplastics in Antarctica, and now remeber that on Mars it's much colder, there is virtually no air, and you have to dig tunnels everwhere, since any sort of prolonged exposure to the surface would irradiate you. Oh, and make sure you do this without any dependence on fossil fuels, except perhaps a few tons every few months.