Doesn't this analysis rest on the assumption that movies are equally good each year? Maybe I suffer from nostalgia (or survivorship bias) but I'm not sure this is a good assumption.
However for certain categories the past was most definitely better. Dramas being the main one. Without a lot of fancy effects the plot was the only thing you had to carry the movie. They also don't beat things over your head and can embrace subtlety in a way modern movies do not.
I also really liked the 70s trend of movies that didn't really have a satisfying ending, you just get to experience the lives of people who don't really improve. 5 easy pieces or The Last Detail fit that category very well.
Everything is evolving.
In old movies actors play often too dumb, just few of them have good examples of how to play a role.
Some "legendary" movies from the past would fail in theaters nowadays without their legendary reputation.
We have more movies right now, more choices, so our demands are higher - otherwise we will not have enough time to watch them all. Also, we have much more choices of how to watch them - age of digital distribution.
Higher demands give higher quality, movies evolve both in visuals and in acting art.
I would suggest an higher standard deviation on quality, meaning that you could reach very top notch standards (your point) meanwhile mass producing total flat works as in music and i presume, all of the arts (my view).
We only want to remember good things.
The "quality" you're referring to, i don't know: you're talking about what features of a sound...fidelity, loudness, dynamic?
Anyway, truly the 80s were the years of trash music...but trash as in metal though.
That's because back then we didn't have the internet. No matter how bad a movie was, people would go see it for at least a week before the word got out how bad it was. Looking up movie times was a pain, you had to plan ahead and check the newspaper. If you went on a date Friday/Saturday night, the most likely thing to do was show up the movie theater after dinner and buy a ticket for the movie that started next, since you probably hadn't heard of any of them, or you would buy a ticket that you'd seen the most advertising for.
There were no ratings to check, no reviews to read, unless you happen to read them in the paper that morning.
Nowadays, they know they have to produce at least something minimally good, because word will get out very quickly that something is bad, and it's super easy for people to check rating when they arrive at the theater, or even before they arrive.