Third option: fade into obscurity as an absolute world power, but become a powerhouse in something like finance and entertainment but not much else relevant to the rest of the world.

Source: England.

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.