Unipolarity isn't the only solution but it's the most apparent one, and also the most terrifyingly totalitarian one.
1) Someone demonstrates a bigger stick. At this point a bigger stick is effective counter-nukes, or perhaps the presence of an alien civilization.
2) What you call unipolarity, although I think both are unipolarity. Mass behavioral transcendence towards a higher form of morality. Maybe achieved via BCI, or improving standard of living, or subtle coercions by AI, or looming threat of climate change prompting transformative cooperative efforts.
The latter was omitted for brevity, due to its unlikelihood. Of course it’s not binary, there’s a bit of each in the other.
There will come a time when taking from others because of the unwillingness to be a creative member of society is seen as a curable mental illness.
I feel the same, but, this is one of those all or nothing things, and the irony is that our own barbaric origins are holding us back from ‘evolving’ so to speak.
We have armies and nuclear armaments because, when it boils down; our species is full of agitated hungry animals, liars and cheats.
You’d need to organize a whole species around a single-world order if you ever wanted to stop wars and denuclearize.