Two-thirds of world's resources 'used up'

According to the latest report from a group of scientists and economists

Here's what intrigues me. By all accounts people breed for the survival of the species, correct?

Why is it then, that people with children are not the biggest defenders/ champions of the earth's resources? It's a bit cruel to hand off a planet that can't sustain them; that is, one that is going to kill them or their children, or not? Or maybe, they don't actually care. What's the incentive to care if you are going to be dead anyway. So likely they don't breed because they are unduly concerned about the survival of the species, then. It must be something else then.