Isolationism will never become so strong that they limit influence to literally the country USA. If anything, they will refocus efforts on the americas, taiwan, rest of the pacific, away from the middle east and the indopac[1]. This means making sure japan, phillipines and taiwan are US allied, and of course that they are the sole influence in the Americas.
[1] which they have taken a lot of massive steps towards recently
That would require a functional economy though, which is why the US been able to do what they've been doing. Without it, I'm not sure how they're hoping to achieve this, but I'm guessing "oil" somehow is involved in the calculation, considering today's actions.
Ya, I find predictions for what would happen in that scenario extremely noisy. Tend to avoid them.
But I do believe that a lot of the US government posturing that if implemented would be bad for economy (which you are alluding to), will not actually happen. A massive revamp of the economy is what the majority voting bloc in the US wishes for, so the winning politician will have to keep signalling in that direction. But actually following through with it, be it in a way with positive or negative effect, is extremely difficult. So I don't pay much attention to it.
[1] which they have taken a lot of massive steps towards recently