Famously, the two main parties switched their roles in the South in the 1960s and 1970s.

It may happen with the unions too. Especially with the blue collar private sector unions. I do not expect this change for public sector unions to happen.

That's your cognitive dissonance speaking; it's a demonstrable fact that Trump took a large part of the blue collar/union electorate in 2016.

But he wasn't appealing to those voters on the basis of increasing the power of unions against rich business owners.

The question is whether those voters wanted other things more than that (and whether Trump managed to deliver those other things to their satisfaction).