I think the closest and most recent example we have is the Soviet Union. The US has a more homogeneous culture and language though. However, I can see states "caring" about themselves more than the union. This might actually turn out good for some states though probably catastrophic for some others.
The USSR was in a far worse position than the US is in. They had to build a wall to keep people from walking out. The US on the other hand is financially irresponsible. It can't afford a global Not-An-Empire any more because its economy is no longer the world's unchallenged largest but it isn't in a bad state in absolute terms. The scale of the challenges just aren't in the same league.
Unless it decides to go down fighting and gets something important blown up it could easily remain a nice place to live for centuries. Nobody threatens the US, it is hard to see how it'll ever be in anyone's interest to threaten them, they can re-learn how to be an industrial powerhouse if they have to. Almost all its real problems are ill-advised domestic policies that can be changed pretty quickly if people decide the situation is hairy enough to be honest in politics.
It might be in everyone's interests for the US to dissolve but it won't be as bad as the USSR dissolution was. It'd probably be a good outcome if they went back to the constitution as intended, depowered the federal government and became a collection of powerful states. A lot of the political tension is because whoever wins the fight over controlling the central government ends up with far too much power.
When I look at the Gerontocracy that rules american politics today, I can't help but remember myself of the Soviet Gerontocracy that ruled the USSR in its last decade or so.
I think the collapse might take place in the shape of something like china: self interested elites, self sabotaging economic policy, a hollowing out economically by outside powers, numerous failed rebellions and devolution of central power due to lack of governability, etc. Aka, a period of somewhat rapid decay.
Education funding per pupil has steadily increased year after year. Maybe we should spend even more, but our problems with bad educational outcomes are mostly due to other factors rather than a lack of money.
Automation has done more to remove manufacturing jobs than offshore by a significant margin. 50-70% of job loss and wage reduction can be attributed to automation.
Even if we bring back all the manufacturing jobs we've lost, they represent fewer positions and automation will continue to be a downward force on wages.
Yes, Gibbon makes for interesting reading. Decline can indeed be prolonged. I suppose it's like how some things get very slowly better over the decades, other things get very slowly worse over the decades.