The 'middle period' of the USA will probably refer to something like post-reconstruction or ~WWII until whatever really sets off their decline/balkanization. Could be nowish.
I think the double whammy of covid and trump was the tipping point for our decline. We're just too close to it to see how quick the decline is in a historical sense. It's only been 4/8 years depending on where you're drawing the line and shits already so much worse