This gets mentioned a lot on Slashdot but, in reality, the number of "good" movies has remained reasonably unchanged each year.
Here are the movies in the IMDB Top 250 grouped and counted by year:
IMDB ratings have a serious problem as far as new movies are concerned, as the latest movie of any reasonable quality tends to get many people rating it a "10" (which should mean it's perfect). It takes a while before a movie settles down to what its real rating should be. This is caused by the "aging" algorithm and number of required votes per year that IMDB uses. It means that a movie that has a lot of buzz will be listed until everybody stops caring about it and it drops out of the list, even if it has a rating that is technically better than movies in the list.
Likewise, there is no reason for movies in the top 250 to be evenly distributed by year. It's far more likely that good movies should be much older, as being evenly distributed by year implies that this year movies have been good enough to push some other movies out of the top 250, which means that the best movies are getting better, which most people agree isn't true. Even, then, there are a lot a problems with ratings being inflated as time goes by. As little as 5 years ago, a movie could crack the top 250 with less than an 8.0 rating, but now some movies are left off even though they have that same rating.
If you use IMDB info, the "Top 1000 Voters" and "Metascore" are far better indicators of the overall quality of the movie, especially if you take into account the number of "Top 1000" that entered a rating for the movie. Basically, these are people who see and rate a lot more movies than anybody else, so even if their score is high for the movie, if a lot of them never rated it, that says something by itself. For example, Star Wars and The Dark Knight are two movies that have both been around for long enough for everyone to get a chance to see them and vote on them, and have ratings of 8.6 and 8.2 from the top voters, with 930 and 898 votes. Both ratings are close to the overall ratings. On the other hand, The Hunt from 2012 (also old enough for such die hard movie viewers to have seen it) gets a 7.3 (considerably lower than the 8.3 from all voters), but only 421 bothered to see it. Django Unchained from the same year, OTOH, gets 710 votes for a rating of 7.8 (still lower than the 8.5 from all voters, but not as much of a drop). The confidence that the rating on Django Unchained is more accurate is much higher. Even using overall votes, Interstellar has less than half the votes compared to the average of the two movies immediately surrounding it on the list, and as such will eventually fall to where it really belongs.