The government and mostly Democrats killed the golden goose and the Detroit auto industry. They regulated, restricted and taxed it into the ground. The auto industry was booming and expanding in the 60s - and then the zippy pinhead liberals decided that cars had to be more ethical and nice, as did their factories. At the same time, trade unions instead became left wing political organizations. And that was the end of the boom never to return.
I think the legitimacy of trade unions ended when they shifted from militantly opposing cheap immigrant labor to militantly supporting it having become Democrat Party groupies. From that moment, unions became the open enemy of their own members.