I get what you are saying, and I agree with a lot of it. IMO two major shifts happened in that time frame - automation and the the societal push that "everybody" has to get a college education. The first one is only going to get worse as we lose more and more of these jobs to automation. That automation fed into the part of the equation that people are replaceable, and I would guess that is a good portion of stigma and treatment of the type of work they do in that time frame. There are more and more of us every year, and there are less and less of these types of jobs. That is a big reason many push for an idea like BIG (I read your other post that you aren't against the idea, just echoing some of your concerns).
That second part, I don't know what to do about. In many threads about poverty, I have advocated for getting back to having more focus on trades and getting people down those paths. Not sure how that gets to a point we want without reintroducing it into HS again.
The one idea that I seem to lean into more that doesn't seem to be brought up much is the idea that the American Dream is eroding for a lot of people. Also what was possible 50-60 years ago is that those jobs would be able to land you a decent living, get a kid through school, allow for a parent to be home, etc, etc. You pull that carrot too far away from people's faces, and their motivation to work will also start to wane. Now how common would it be for people who work in similar jobs to support their whole family? Is their purchasing power similar than 50-60 years ago? That's something else that is missing - it was a secure, valuable job that gave people a sense of pride because they could accomplish something for their family. Now it's a job that can be replaced at the drop of a hat by a robot, little security, and probably requires a 2nd job or the other parent to get a job to make ends meet.