There are plenty of signs that the US could be in the midst of its own empire failing. Whether that actually plays out is something we wouldn't know for decades or centuries, but there sure are a lot of parallels to failed empires of the past.
So far it seems like it'll be China on account of there being no other country or bloc able to compete at that level, but that would be very unpleasant for us westerners.
If the US and China really were to get into direct conflict, the bigger question may be who on earth was least impacted by nukes rather than which of the two countries "wins" (in quotes because we all lose in war).
Unfortunately I don't think the US would go down quietly if the empire really is failing. Another major war wouldn't be out of the question, clearing the board for a lot of change to take place.
Without a war though, and one can only hope this is the case, I don't see China pulling it off. I both don't think they really have that motivation, and I don't expect they really could transition from such a totalitarian state to an empire driving anything near the level of US dominance.
When the US empire fails, and it has to eventually even if we aren't currently walking that path, the question is whether it falls apart or fails violently. You could point to many parallels with the Roman Empire and guess that it will just fall apart, even if there are fringe wars along the way.
They already did, and the US had to blow up a pipeline to keep them in line. But that capability won't last forever.
There's a lot rolled up in such a small idea. What makes another population barbarians, and how is it so different from the US? Assuming there are groups of adversarial barbarians around the world, why should the US step in to "fix" it and how can they even define what the fix is?
Irrespective of the origins of America, that is a categorical difference in the scale of death and destruction. To Americans, that is barbarism, but it's possible that Europeans do not see it as such and perhaps see civilization as inherently violent.
They are foreign to us here and speak different languages so perhaps their true minds are forever opaque to us, but we can see their actions: and that is repetitive quarrels with their neighbours with frequent attempts at ethnic extinction. From that, some conclusions can be drawn.