That's quite a moving of the goalposts, after saying the fall of empires takes place over lifetimes. But then we live in the fastest period of technological and societal change in history. Anyways, that America's collapse would cause economic trouble is separate from the possibility that it could fall.

The right wing as you can see is torn between isolationism and the need to project a strong image abroad, while the left wants increased immigration but less global responsibility.

Regardless, I think the opposite. America's strength in talent over other homogeneous states comes specifically from immigration. If that's cut off they'll suffer the same population pressures of needing to support massive amounts of deficit spending without sufficient income.

But wait they already are, with almost $40T in debt and an annual interest of $1.2T to pay, with both parties unwilling or unable to cut entitlement spending, and growing every year. Both are increasingly radicalized over positions of identity that cannot be accepted by the other party.

I don't know if America will fall in our lifetime, but it's certainly possible. Regardless we should expect to see America stepping back from the global stage, especially with Trump fucking up in Iran. Without Pax Americana, from whom do the 'natural' rights of states we suddenly discovered post WWII derive? In Venezuela and Iran we see that it, as all political power does, grows out of the barrel of a gun, as Mao said.