The participation rate has definitely been in decline.
There is no evidence that the cause of this is a lack of manufacturing jobs.
Rather, he official definition of workforce age is 16 and older. There is no top end (oddly). So obviously the baby boom in retirement age will shrink participation rate per the graphs and this has been known for some time. Big numbers retire.
Then there is health. US consumption of pain opioids by working age people has exploded. One half of working age men take pain medication, and 2/3 of that total are on prescription pain medication. That shrinks the available pool of men of that age to work in factories.
And so. There is no pool of non tax paying people poised to jump into these factory jobs and pay tax revenue.
Says who? A cretin like you?
Stagnant wages and a low birth rate are signs of: