What seems least plausible about this to me is that a long period of decline like that (basically what happened to France and the UK) wouldn't result in something like a civil war or other split in the US.
Based on observed uprising or civil wars [1], to have one of those you need (1) stark mass poverty that borders on famine, i.e. people really scrapping by, and (2) Youth! It's much much easier for a younger population to revolt.
And, no offense, but relative to the rest of the world Americans are kind of old and kind of fat.
And, no offense, but relative to the rest of the world Americans are kind of old and kind of fat.
So: probably not.
[1] Sources:
https://www.forbes.com/sites/jerrybowyer/2013/07/18/youth-in...
https://www.fragilestates.org/2012/11/25/causes-of-revolutio...