... no, most of it was just due to automation and advances in production. American productivity in terms of the stuff those jobs used to come from has largely either not budged, kept growing, or barely slowed down. And yet the workforce continues to shrink, and it's not going to stop. Outsourcing is a thing but when it comes down to it the core issue for unskilled workers is that they're increasingly just not fucking
needed anymore. Not that the job can get done cheaper elsewhere (though it's definitely an issue, it's not the major one), that the job can't be done cheaper by humans period, or that one person is now doing the job of several (dozen).
Even if you magically ended all outsourcing overnight and produced absolutely everything locally (and it somehow didn't smash the economy into little pieces in the process) those jobs
still wouldn't be coming back to nearly the extent they used to, and even if it actually slowed things down they'd still be bleeding out. Literally the only way to fix that would be burning down modern factories and infrastructure, or assuming direct control and instituting employee productivity caps or something mad like that. What has mostly cost these folks their jobs isn't cheaper labor elsewhere, it's increased productivity per worker. Even if you artificially increased the price of labor you'd still be losing jobs compared to even a few decades ago, never mind during the height of factory employment.
Effectively what's happened/happening to former blue collar/manufacturing work is the same thing that happened to farm labor back when agriculture started getting better equipment. We've still got roughly the same (or better) muscle when it comes to actual
production*. We just don't need nearly as many people to do it.
*The net domestic growth, by the by? Averages out to
~3-4% per annum since 1920. Manufacturing output has been increasing the whole goddamn time the workforce has been shrinking. Roughly 370% increase in production over the last ~century.