I think the term "Hollywood" is really the key here.

The '50s was really the zenith of the Hollywood studio system. Studios were vertically integrated, owning not just the means of film production, but also the actors, and even had huge amounts of leverage over the theaters themselves. You want to show the new Cary Grant film? It's yours, but you also have to buy these other 50 films, most of which are probably not good.

However, in 1948 the Supreme Court ruled that the studio system was an illegal monopoly, and slowly the entire structure crumbled, and by the 1960s, studios were a shell of themselves in term of their overall scope and influence. The industry became much more director-oriented with the rise of "New Hollywood" by the late 1960s, pretty much replacing the studio-dominated landscape.

Films definitely did not get worse after that point, but the term "Golden Age" usually refers to the fact that this entire system enabled studios to constantly pump out film after film. Sure, many were bad, but in terms of pure output, Hollywood was never healthier.

Also, I'd say that the really definitive film movement from the 80s/90s was the rise of independent cinema, rather than Hollywood itself.