The US has too much going for it in terms of population size, geography, natural resources to fade to the extent the UK has. At least in the near future.

That's the unconditional surrender scenario. As long as most of the stuff you buy is from abroad and you're willing to sell whatever you still produce that has any value, nobody actually cares enough to threaten your sovereignty.

China has no interest in a war, at least not in Europe. They are quite happy to protect their borders and then show the world their superiority through economic and cultural means.

I'm not as certain about Taiwan, but I believe that the CCP would rather have a peaceful "reunification" there as well. As long as that's not possible, it's probably sufficient to be ambiguous about the whole deal as we've been doing thus far. Remember, Hong Kong fell because the lease expired, even though it's pretty unlikely the UK still had the muscle to hold it by force in its later years.

Instead we're probably heading towards another cold war, in which trade, diplomacy and quality of life plays a heavier role than raw power. In this scenario, AI doesn't actually help anyone gain a competitive advantage since it will be stolen almost immediately (it took the russians what, two years to steal the A bomb?).

And with all other things being equal (to be clear, right now they're not, or at least not in your favor), you'd still lose because you're outnumbered 4 to 1.