People often claim survivorship bias for such things, and it may sometimes be true, but I think it often misses the point. It may be true that overall, the landscape of an art form wasn't much different than today. However, it is obviously plausible that the "highest highs" would be higher in some period X than some period Y - it would be actually much more surprising if an art form were of uniform quality across many decades.
I don't have enough cinema culture to comment on cinema specifically, but I believe this is pretty obvious in music. Comparing things like Beethoven's 5th and 9th symphonies to any modern music (especially if comparing only the main themes, given today's preference for very short form music), it's obvious that there is nothing similar, and even modern audiences generally recognize the superiority of the older one.
As a more focused comparison, it's also obvious and largely uncontroversial that the amount of good rock music being produced has plummeted since at least the 1970s-1980s. There are still a few good bands (Rammstein has been an unexpected highlight for me), but compared to a period when you had Pink Floyd, Led Zeppelin, David Bowie, Black Sabbath, Queen, Deep Purple, Metallica, The Rolling Stones and a good many others, it's obvious that something has gone down in the highest highs of music.
Well, I myself is an avid concert goer (before Covid at least) and I probably know more about classical music than I do about classical cinema :-) Although I mostly prefer music written c. 1900-1950 rather than Beethoven or his contemporaries (what a lot of people don't realise is that there were a lot of changes in classical orchestral music over time; the difference between, say, Mozart and William Walton is no less than the difference between Beatles and Metallica, probably even more so).
And indeed, the large & majestic sound of a symphonic orchestra has no parallels with the modern music. But on the other hand I think we can easily compare classical chamber music with jazz/tango/rock. And in this field I'd rather listen something like [1] than any classical quartet/quinter regardless whether it was written by Beethoven or by Shostakovich.
No, obviously out of ~300 films produced in 30s and 40s each year, 90% or more were dross. What I've meant is:
1. The best films of this period far surpassed my expectations from the technical point of view. And there were a lot of decent-to-exceptional films produced at the time; I could name at least 50 american films worth watching from 1930 to 1950. In comparison, I could hardly name 10 films from 1951 to 1960 that are at least decent (and yes, Paths of Glory, named below, is the best).
I think it was mostly due to the fact that all personnel, connected with the creation of a film at the time were still largely pioneers at the field and they had all possible expertiese in it (films in the 60s and especially in the 70s became noticeably more amateur; 50s suffered due to McCarthy). Movies were still relatively new and there were a lot of innovation in it each year. On top of that, it was a time of the Great Depression and high unemploymend. Hollywood were one of the better off industries and so were able to easlily atrract best of the best.
2. Even B-movies from rich studios had high production values. From the same 1941 I could easily recommend for example The Gay Falcon - Irving Reis - RKO/Nothing But the Truth - Elliott Nugent - Paramount/Charley's Aunt - Archie Mayo - 20 Fox. All are relatively simple, but well worth the watch.
3. What's more important, the 30s and 40s cinema had its own unique style and dynamic, very different from later decades. I'd say it was closer to Imre Kalman and Franz Lehar operettas, rather than more convential movies we are used today. It was, if I may say so, a thing-in-itself, hardly comparable with what came later.
Survivorship bias? I'm sure there was as much schlock produced back then as there is now, it's just we elevated the best and have forgotten the rest.
Revisiting the bad stuff is the entire premise of MST3K:
* https://en.wikipedia.org/wiki/Mystery_Science_Theater_3000