It could go back to the laissez-faire system that was present when all the European powers were running around basically doing anything they wanted. (It should be noted that the US wasn't as isolationist as the propaganda would have you believe: how else did they get pre-revolution Cuba, PR, Guam, Hawaii, etc.?)
I think these systems align to a certain power dynamic. Because of national sovereignty, its already as close to a laissez faire approach as you can get, yet there’s a hierarchy across virtually any dimension you want to measure. So if the US loses, for example, its economic hegemony another country will take its place. Same with military or any other measure.
I don’t think the US has been isolationist since before the Spanish American war.
China's Belt and Road program seems to be establishing a competitor hegemony in the eastern sphere of influence, rather than directly replacing the US. They're building a lot of infrastructure in Africa, expanding their already massive supply chain and working effectively towards surviving a disconnection from the West.
I agree that US hegemony isn't gone yet, but we've lost so much of our manufacturing and materials sourcing that we'll soon need them more than they need us.
I think you can make a strong case US hegemony may be in decline, but it’s harder to claim it’s gone