But the problem with thinking of Mars as a fallback planet (besides the lack of oxygen and air pressure and food and liquid water) is that it overlooks the obvious. Wherever we go, we’ll take ourselves with us. Either we’re capable of dealing with the challenges posed by our own intelligence or we’re not.
It is a trick of the language to think of humankind as a unitary "we." We are not one. We are diverse groups of people attempting to build different sorts of communities, on Earth and off. Pilgrims left for America and founded a nation that changed how state and religion interacted. Likewise, groups from earth may leave certain problems behind, even as they encounter others.