I'm guessing those companies would have died of because they couldn't compete with cheap overseas labor

And none of that would change the fact that other countries are now able to buy goods they once bought from US manufacturers from other countries.

China, Japan, Germany, and other modern industrial powerhouses that were decimated after WWII are now competition for US manufacturers. Even if the US went full on protectionist and kept their own factories from offshoring, that wasn’t going to prevent China (or Germany, or France, or whomever) from pursuing its own industrialization, and then going after countries that were buying American in the mid 20th century to buy from them instead.

A good example to look at right now is Australia — in the 1950s, they were dominated by American vehicles, either importing them directly from the US, or via locally built Holdens (which may have been manufactured locally, but the company was owned by General Motors — the profits flowed back to the United States), with some small British cars thrown in. But today only two American brands even make it into the Top 10 — Ford at #3, and Tesla at #8. All of the other brands are now primarily out of Asia (Toyota, Mazda, Nissan, Mitsubishi, Kia, Hyundai, Isuzu).

The US “not offshoring” manufacturing wouldn’t have changed this. And the story is the same across all of the rest of the world — back in the 1950s, the world was buying from the US (or to a lesser extent Canada and the UK). Today they don’t have to — and US protectionism was never going to change that. Forcing American companies to stay in the US and only hire Americans would have just made life for Americans more expensive, and wouldn’t have changed the fact that money that had been flowing into the US during the post-war era had more or less dried up once the major economies in Europe and Asia had re-industrialized.