The problem we have with identifying the potential fall of America by using historical examples is that we are currently living in an era unlike any other. This era began in earnest around the same time that the US became a superpower— the era of instant mass media.
I can even show you what I mean: what would you call today's civilization epoch? When we talk about civilization collapse, we tend to refer to a specific dominant empire. Civilization collapsed with the Romans, did it not? And we know of the Bronze-Era Collapse. But if America collapses in the next 10 years while, say, China and India keep their heads above water, would you say that civilization has collapsed? Of course not! Civilization today is too global, too planetary.
This is unprecedented throughout history, and it's why even the inevitable fall of America may not be exactly like what we saw with the Romans. There is genuine interest in making sure certain regions of Earth are highly developed. America is one of those regions; there remains tens of trillions, possibly even hundreds of trillions of dollars of materials on the continent. If and when America falls, it's probable that this will give Chinese venture capitalists/communists opportunity to move in and redevelop the nation, but under a red flag instead.