I think there is a simpler explanation for the fall of empires: They are built on assumptions about the world in which they are founded, and they come with coping-mechanisms to deal with the kinds of problems that their founders expect them to face, based on their own experiences and their understanding about what went wrong in previous empires. In this way they are like a code base, built to be easy to extend in the ways where extension is expected, but rigid in the directions where it is not. But the world changes, and eventually it will change in ways the founders could not have predicted, and which the empire is incapable of fully adapting to. Not only that, but as the empire grows, power is concentrated in its midst, so it becomes more and more lucrative to game the system, to find ways of exploiting the rules and institutions of the empire to gain access to this accumulation of power. (In startups this is similar to when the MBAs start running the company and all the creative people leave). These adversarial attacks further break down the coping mechanisms that were supposed to help the empire adapt to a changing world. It leads to consolidation and inflexibility. And as the center of power increasingly attracts those with no interest in the sustainability and well-being of the empire, unrest increases as the common man loses faith in the empire, and its ability to deal with the issues of today. The empire is now in crisis, and it will either adapt through a massive change (a civil war that is swiftly resolved, a 'New Deal' or similar) that 'resets' its alignment, and makes it capable of change in new needed directions (not unlike a pivot in startup parlance), or it will fall apart (typically through a civil war that never really ends).

The US has managed to survive several such crises (e.g. the Civil War and the Depression) but there are many ways in which the coping mechanisms that its founders created have been hamstrung. The 2nd amendment made perfect sense when the biggest perceived risk to the wellbeing of your citizens was a foreign tyrant occupying their land - but today it is far, far more likely that you will be shot by your neighbor. Guaranteeing free speech makes perfect sense, but the founders never predicted a world of multimedia broadcast, where the person or company with the most money can drown out the speech of everyone else. Letting everyone be free to pursue happiness on their own is a great principle, but it never predicted a world where people and corporations could be rich enough to suppress social mobility as much as an aristocracy does. The three branches of government were supposed to provide checks and balances on each other, but this breaks down when massive partisan polarization puts the executive and legislative branch in bed with each other, and they join forces to appoint politically motivated judges. The electoral collage, a mechanism that was supposed to prevent the majority from ignoring the will of the minority instead became a tool for the minority to ignore the will of the majority. The list is long.

As an outside observer, I really hope the US can pull through its current crisis (the world could do with fewer empires, but I don't particulalarly want the stage to be left for China or Russia instead). But as far as I can tell, it requires not only that the US changes, but also that it changes the ways in which it can change. That's a tall order considering the current state of affairs.