One strange thing about writing is that there’s always a way to make a piece of work a little better. But sometimes we have to kick our work out the door, believing that it is now “good enough”.
In my sense of history, there really isn’t a total vacuum of government as the old empire fades away. New governments emerge; seldom is there a total breakdown of civil order. Even dictators have to be mindful of how far they push the citizenry. And sometimes the “barbarians” are better the former governors.
If the USA does not get rid of all political parties, I think your scenario is very plausible. The parties no longer can hold this nation together. The trouble is, in my mind, if these smaller nations continue to use political parties, these nations too will be short-lived. Average Americans will, sooner or later, have to play a bigger role in governance than voting and sending $100 to their preferred political messiah.