Did a quick google search on number of movies made each year; actually it looks like modern movies versus old movies have about the representation you'd expect if quality of movies was essentially constant (the number of extra modern movies on the list corresponds approximately to the number of extra movies made.)
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: