The US did fine after WW2 isolating itself
The USA did not isolate itself after WW2. Exactly the opposite happened.
The idea that the US needs Europe is just hilarious.
About 11% of our GDP is based on exports; about 15% is based on imports. You really want to tell me that the USA doesn't need the trade with the EU and China? How do you plan to compensate for the loss of that trade and the loss of related jobs? Increase domestic consumption of domestic goods? Good luck finding the money.
Cut the US off? Go ahead, like Russia has proven, the US would prove even better at surviving alone.
The US is the largest economy in the world, Russia only survives because the Chinese are helping them.
What happens when the US stops the idiocy of lending itself money and just prints off enough to pay off all the debt?
The moment the US$ stops to be the reserve currency of the world, the USA will be bankrupt.