Decline of unions occurred because too many unions got greedy making themselves rich while driving labor cost up too high without getting the productivity out of their workers to justify it!
And making it impossible to fire unproductive people, which incentivizes laziness. The best partner I had in an aerospace shop never made a chip. He would collect the cutters from the tool crib, preset them on the workbench, and load the fixtures. That was his day. He'd sit at the workbench and read his book.
I'd come in on swing and machine the parts. KC-135 landing gears. Big 15-5 forgings. We had an understanding. He wouldn't backstab me on dayshift and I'd do all the machine work.
But this is what really caused the demise of private sector unions- they ceded their power to the gov't. In the 1920's, there were no labor laws. Unions were attractive to workers because it was the only power they had.
Over the years, all the benefits you received from being in a union were coded into law (at the behest of the unions themselves). 40 hour workweeks, overtime pay, minimum wage, paid holidays, health insurance, workman's compensation, unemployment, retirement, etc. It's all there. The unions have very little to offer now that isn't already free (from the perspective of the worker).
I still don't object to private sector unions. It's okay to have the greedy capitalist on one side of the table and the oppressed worker on the other side. If one side gets too unreasonable, the other side can walk away. Workers can just quit, and factories can be relocated.
This doesn't work with public sector unions. There is no capitalist on the other side of the table. And you can't just relocate a school or a public transit system if the unions demands can't be met.
Private sector unions organize against capital. That's okay- capital can move. Public sector unions organize against the taxpayer. But the taxpayer doesn't have a seat at the table.