We won't see any significant change in the world political situation that recognizes the US as the world power until there's another country or faction that is clearly the replacement

That's where I think the Roman Empire analogy is helpful - because it suggests that this might not actually be true. I imagine if they'd been thinking about this back in Roman times they'd have been speculating about Parthia and so forth. But it wasn't like that.

Rome collapsed from within because of the contradictions it involved - being simultaneously a Roman Empire and a European (and beyond) empire. It didn't fall because the barbarians conquered. The barbarians conquered because it was collapsing. No one took over. And the rump of the Empire survived another thousand years in Byzantium, increasingly irrelevant.