It can be the case that people didn't feel that unions were a net positive from the 1960s~2000s and now, starting from somewhere in the late 2010s, have begun reconsidering whether unions are net positive. All that needs to change is conditions altering which make unions seem to be a net positive.

Sure. It can even be the case that unions were a net positive before 1960, too. It can even be the case that the gains achieved by the unions are what made conditions less terrible from ~1960 to ~2000, and that the absence of the unions is what allowed conditions to get worse after 2000. (I don't think that's the whole story, but it's a defensible reading of events...)