But on US mfg not having declined in raw terms: are you saying e.g. that roughly as much steel is produced today in Youngstown, Pittsburgh, etc. as was produced in, say, 1968, just with many fewer workers and fewer plants? That would surprise me.
Via. I'm not sure about steel in Youngstown per se, but overall the US manufactures more now than it ever has. As a percentage of global manufacturing output the US hasn't declined too much either. Until the subprime crisis/Great Recession the US had stayed around 25% of global manufacturing output since 1975.
Despite that downtick, the US was still the largest manufacturer in the world in 2009 (more graphs and discussion at that link). The view that the US just doesn't produce anything anymore is common, but it's wrong. We still produce quite a lot, in both raw terms and as a share of global output, and we do it with fewer workers than ever before. That's bad for the unneeded workers in the Rust Belt, but not for the broader economy.