This article asserts that we film snobs are wrong about "today's movies aren't as good as they used to be". I tried to conduct an experiment on imdb.com, to compare top ten grossing movies for each decade year (1950, 1960, etc). But imdb only computes top-grossing figures as far back as 1980. For 80, 90, and 2000, I found that only 2000 had a top 10 film (Gladiator) that I consider excellent. Everytyhing else I found to be unwatchable awful, though I suppose I should give 1980's "Ordinary People" another chance.
imdb does calculate for all years a ranking by "total votes" of its voting visitors. Here's how I rate each year's list, by fraction of "top ten vote-getting films" that I rate as excellent:
2004: (20%) I didn't include Million Dollar Baby, Garden State, or Crash, though I'm open to adding MMB and Garden State.
Thus I must conclude that I really do think that the quality of films overall has diminished, but a lack of box office statistics before 1980 makes my study non-definative.