Hundreds of millions of rocks floating around our solar system...all we have to do is miss one of sufficient size and boom, we’re done. If not as a species then as a civilization.
Mars is the backup plan. Eventually it’ll be the frontier and finally another epicenter of humanity. Few things are as important.
Mars lost its atmosphere a while back. Not sure Mars is a tenable solution. You’d be way better off living in the cloud layer above Venus.
That IS doable.