there is anecdotal evidence to support your hypothesis - when I was in graduate school, I became friends with a few older professors who told me that when they started teaching, grade inflation was not a big issue and freshmen came to college prepared - they could write properly, make reasoned arguments, etc. After about the 1990s, all of a sudden more and more students needed remedial courses to do minimal college work and professors were pressured to give higher grades than were deserved.

I've seen this play out in other areas - Scientific American, for example, used to be a high level journal when I read it regularly in the 1980s. When I picked it up 20 years later - it had been dumbed down to the point that it was no longer worth reading. I used to need to ponder and look things up to understand a lot of the articles - now I find them juvenile.

Same can be said of various forms of art, as well. Music used to be more mathematically complex, for example. The news used to be more nuanced and deeper. People like Tesla were celebrities, not the Kardashians. We sent a man to the moon using slide rules and built skyscrapers with what would now be considered primitive technology.

I think Idiocracy is prophetic.