I think that there are good movies from each year. But some are so specific to that particular time and year, that yes, if you go back 10-20 later and watch them, you think, "Wow, that isn't a good movie".
To me, it should be more, "That was a good movie when it was made, but today has different interests, trends, different time of your life, etc, so it is still a good movie, but not today" (Hope that makes sense)