I'm sure that demise has nothing to do with the change from unions literally fighting armed strike breakers on the streets to unions being heavily regulated bureaucratic entities that are on first name terms with the corporate owners that used to hire the armed strike breakers.
By legalizing narrow avenues for union conduct, we have also heavily disincentivized any behavior outside those avenues. The "fat cat" union reps that live off your dues without contributing anything meaningful to your rights are ultimately an intentional creation of this de-radicalization. Incidentally the Red Scare helped quite a bit by making unions scared of seeming "too leftist" when the existence of unions itself is born out of a leftist understanding of class conflict (not "class" as in how much money you have but "class" as in whether you have to work for a living or people pay you for what you already own).
By legalizing narrow avenues for union conduct, we have also heavily disincentivized any behavior outside those avenues. The "fat cat" union reps that live off your dues without contributing anything meaningful to your rights are ultimately an intentional creation of this de-radicalization. Incidentally the Red Scare helped quite a bit by making unions scared of seeming "too leftist" when the existence of unions itself is born out of a leftist understanding of class conflict (not "class" as in how much money you have but "class" as in whether you have to work for a living or people pay you for what you already own).