The world will be bipolar USA and China, and we'll see. It should work out ok since China has a massive population shrink problem, plus it is not making any friends around the world. The question you always have to ask is 'who are a nation's allies?' China doesn't really have any allies. None. North Korea sometimes.
Whereas the USA has as allies all the freest, richest, best nations on earth. So that's good.
The worry about the US is its politics...Clearly right wing politics are ripping the nation apart, as right wingers attacked Congress and have replaced muslims as the main terrorists active in North America.
And our lack of population growth and anti-immigrant swing is suicidal.
As Hardloper points out, we have a lot of natural protection and benefits. If we are defeated, it will be at our own hand.
Abe Lincoln had it right:
"All the armies of Europe, Asia and Africa combined, with all the treasure of the earth (our own excepted) in their military chest; with a Buonaparte for a commander, could not by force, take a drink from the Ohio, or make a track on the Blue Ridge, in a trial of a thousand years.
"At what point then is the approach of danger to be expected? I answer, if it ever reach us, it must spring up amongst us. It cannot come from abroad. If destruction be our lot, we must ourselves be its author and finisher. As a nation of freemen, we must live through all time, or die by suicide."