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.