Very true, but I also can't (with the exception of A Clockwork Orange) put them in the same category. I saw Eraserhead at a midnight show at the Waverly many, many years ago then again last week. Pretty ridiculous.
You're right that there have been plenty of great movies made post 1950 it's just that the sheer volume of films coming out of Hollywood prior to TV and the breakup of the studio system made it statistically inevitable that there would be so many more great ones made. It's probably true that there were many more stinkers as well.
If you look only at American films, sure, but there was an explosion in worldwide cinema during the post-war years. Italy, Japan, Russia, Australia, France, etc.
And you either “get” Eraserhead or you don’t, same goes for all of Lynch’s films. I didn’t know what to make of it at first, but on reflection I consider it a masterpiece.