Once all empires reach a certain point of decline, separatist movements are inevitable. It happened to all empires in the past and the US will be no exception. Well, I suppose the US could always murder-suicide the world with nukes when it's at a point of irreversible decline and it doesn't want China to take the throne.

So assuming the US doesn't start a hot war with China, it will eventually reach a point of decline where the federal government has only nominal control over its territories. There might be a rump state and it might still have a seat in the UN (or whatever equivalent of the UN), but the US will functionally no longer exist.

Current secessionist movements are just memes. The instant California or Texas even secedes from the US, the US will simply sanction the state to oblivion and politically isolate the state. Without trade or formal state recognition by other countries, how would that state even function? No state is self-sufficient, so how would people in that state even survive without trade?