There's another bias, where things are overrated because they're unusually good for their time. For instance, "Battleship Potemkin" is often considered one of the greatest films ever made because it was so innovative for its time, despite the fact that when you watch it now, it's easily a shitty propaganda film. It may have been a greater achievement, but as a film, it's somewhat lacking by modern standards. Likewise, MASH and Seinfeld were groundbreaking, but largely because of MASH and Seinfeld, we can and do make better television series today. I think the two biases largely cancel out.
This has some interesting consequences. For instance, in the restricted realm of science fiction TV, it's clear that Star Trek: The Next Generation is strictly better than the original Star Trek, just as the recent Battlestar Galactica is strictly better than TNG, because these series responded to and innovated on each other.
On balance, I still wouldn't say it's necessarily true that newer series are better than older series. It's still rare that a TV series tops The Prisoner, for instance.
This has some interesting consequences. For instance, in the restricted realm of science fiction TV, it's clear that Star Trek: The Next Generation is strictly better than the original Star Trek, just as the recent Battlestar Galactica is strictly better than TNG, because these series responded to and innovated on each other.
On balance, I still wouldn't say it's necessarily true that newer series are better than older series. It's still rare that a TV series tops The Prisoner, for instance.