Nature will be wanton, cruel, perhaps overshoot. It will be ugly and horrific. We will not be left in great shape. The only fairness it will possess is unpredictable randomness.
Or, we can do it. Harsh. Belt-tightening. Unpleasant conversations. Lengthy debates about worth. Whole societies wanting their turn at the trough. Deprivation. Lifeboat politics. The Cold Equations in a hot, hot world.
Everybody who decides that what they need to do is kill extremely large numbers of other people can just off themselves instead.
And so the scavengers will feast and the plagues will bloom again. And because no-one will agree, we will have little local wars over resources. Maybe someone will grab a warhead or two. Technology still keeps happening, so expect at least one plague to be targeted. And some group will have a "if we can't have it, nobody can," so a self-irradiated country will get to keep a resource out of some other group's hands. You'll get real genocides, then.
Nobody wants to participate in the one way of getting their hands bloody, pretending that they won't get bloody later on, up to the elbows.
Because I don't know what the solution is but I do know that the alternative is going to be no fun at all. One possibility is that we all end up agreeing that we should at least try to reduce our numbers. My parents brought me up with 'replacement' numbers so that we'd end up with a steady population. I'm not so sure if that was wise. And I'm aware that there are many predictions out there that our numbers will eventually top out (they will, either because we do it ourselves, voluntarily or because our resources will run out, an exponentially expanding population in a fixed environment always ends in collapse).
But unless we manage to reduce our impact on the environment in very drastic measures I don't see a way for the increased consumption and eventual survival of our societies as we know them today. In another comment someone offhandedly asked whether we should all want leisure yachts or not. The obvious answer to that is 'no'. But we probably also shouldn't all want vehicles and the ability to transport ourselves to the end of the world on a whim. Hard choices are ahead of us, the time for easy answers is long behind us.