Well, we can't count on the entire rest of the world having all their factories bombed and being the only nation with reasonable manufacturing capability all the time.

The US was not an export-focused economy in the 50s and 60s. In fact trade as a percent of GDP bottomed out in those decades. Arguing that the prosperity for the average person was only "sustainable" in an environment where there wasn't foreign competition is an argument for tariffs.

https://www.stlouisfed.org/on-the-economy/2020/march/evoluti...

In what way would the citizens of the US have been worse off had the rest of the world had reasonable manufacturing capability? I'd have thought that either it'd be the same, or better, because same production + trade.

It was the US benefiting from its position on the world stage as to why that was possible. The US was never and is not going to retain it's position on the world stage, and indeed under Trump it is radically declining.

The US is going to normalize with other western nations as we enter a multi-polar world.

The problem is Republicans don't accept that and think they can return to the 1950s.