>Just try to think about this: how bad do the conditions on Earth have to get before it makes sense to try to move to a planet that is essentially uninhabitable? Where even in the best scenarios we would live really harsh lives after a long and dangerous voyage.

The point of colonizing Mars (for Musk) is not to move there when Earth becomes inhabitable. It's too remove the single point of failure for the species that Earth currently represents.

With a self-sustainable (even if harsh) Mars colony, a nuclear war on Earth no longer necessarily means the end of humanity.