When there is nothing that can be gained from others

That will always exist. If everyone has enough resources, we'll compete for other stuff like status symbols. That's just how human nature is, we're never content and always want to progress relative to those around us.

And no, human population doesn't need to go down. Every time we get to some number scientists pick for a maximum, we blow past it and figure out how to sustain the new population. I don't see any reason for that to change, we're remarkably good at solving problems we create, though frequently later than is comfortable. I don't know where you're pulling that 528 million figure from...