The music analogy is a bit tricky because we've had a lot longer to explore music than we have to explore movies, and movies are harder production-wise. Modern movies are better than past movies for reasons more like the reasons that music after the pianoforte was better than music before the pianoforte, rather than the reasons some people prefer Michael Jackson to Louis Armstrong.
I'm really not discussing the absolute value of old movies vs. new ones, I'm definitely not expert enough on the subject. I was rather trying to point out other kind of effects I thought played a role in explaining the pattern pointed out in the OP. I used the parallel of music because I thought some of these effects were relevant in both cases.
IMDB top 250 list is dominated by old movies, which conflicts with my perception (shared by majority of people as far as I can tell) that new movies are far better than old movies (comparing either top with top or average with average).
I have a simple theory why IMDB is wrong: