I don't won't to rant politically but I think you are confusing Donald Trump's America Alone with all previous Republicans and Democrats. The idea of us abandoning all alliances and deals for ???? is relatively novel and stupid. The brand of tax cuts and austerity (which are republican coded in my opinion) are what destroyed the UK economy and greatly damaged the US economy (we were more obstinate to full Austerity than any EU country the EU made bad economic decisions in required balanced budgets with rigidity). But we never did abandon our allies completely and step of the world stage. This is what sets a republican like Reagan apart from Trump. I think they are both terrible on the economy but Trump is just a disaster for US alliances and soft power. If the role were swapped than I think their is an incredible likelyhood we would have lost the Cold War. Its just not the same republican party its far far worse strategy.

I know we also argued that WW2 is what allowed the US to coast but I'd argue that New Deal programs that made demand and things cheap (like electric think TVA). This arguablely fuels American industry as it rushes to fill in the WW2 demand. All this investiment happend before WW2 even started and I think doesn't get nearly enough credit for allowing the US to 1 not become facists and 2 allows the US to step in and support the large demand of ww2 and post war. If we didn't invest in the nation we would not meet post-war demand and be much poorier today.

Finally, I don't want to be nilhistic or depressed if we can observer that we did things better at one point we can still choose a future that is better regardless of what mistakes we make today. We can make better choices albet it limited from today's options that will actually allow the US to raise people higher out of poverty. I don't agree with the idea of betting against Americian ingenuity in the long term it tends to lose.