Just one of the fun ones https://slate.com/technology/2015/04/perchlorate-in-martian-...

Wiki has other details https://en.wikipedia.org/wiki/Martian_soil

In any case, point is: We need to stop being a threat to ourselves, and hoping that another planet, for which we're not evolved to survive on, is a better choice feels like the wrong path. Humans are amazing at surviving... on earth. Because we evolved to survive here.

Having a "plan B" is not a bad choice, because a lot can happen that we cannot prevent. However we can't resettle everyone. Most will die. Plan B is to continue the species, but not necessarily to actually save us.