The threat from asteroids is that years of perpetual winter shut down our agriculture for years or even decades. But even in the case of a really massive, Chicxulub sized impact, earth will remain more hospitable than mars. The temperatures will be milder, sunlight at the surface will still be stronger, the air will still be breathable.

If we can build a sealed habitat on mars capable of recycling water and growing its own food, we can do the same here on earth. Just give it a dedicated fission reactor to keep the greenhouses running, distribute half a dozen of them around the world so that they can't all die to direct impacts, and fortify them heavily to keep the starving masses out.