> so who says the US is going to continue to be more-or-less an ally and China is going to continue to not be one?
Geopolitics is much less unpredictable than "the future." Even the current administration is not so short-sighted to alienate the US to the point of having no powerful allies. If Europe is not considered an ally (it is, not "more or less," it 100% is), where would the US turn? Even if you take a scenario where Trump and Russia are allies, and the US positions itself closer to Russia, Russia's allies are anathema to US interests and ideals, and will be for the foreseeable future.
I just can't see any reality outside of borderline sci-fi where the US is not a close ally to just about every Western European country.
One could ask why the US and european countries are allies. I think it is because both are technologically advanced, both are rich, both are societies with human rights, both are democratic. (Relatively speaking, at least.) These things can change. Now that I name four factors it may sounds like a lot would need to change. But the first two as well as the second two are closely related. And at some point the former two are also related to the latter two. If China wants to keeps developing it will at some point need to improve its human rights situation as well.
Geopolitics is much less unpredictable than "the future." Even the current administration is not so short-sighted to alienate the US to the point of having no powerful allies. If Europe is not considered an ally (it is, not "more or less," it 100% is), where would the US turn? Even if you take a scenario where Trump and Russia are allies, and the US positions itself closer to Russia, Russia's allies are anathema to US interests and ideals, and will be for the foreseeable future.
I just can't see any reality outside of borderline sci-fi where the US is not a close ally to just about every Western European country.