I think you can debate forever about best and worst periods, as well as genres, but the thing that I notice is that the rate of change in popular music seems to have slowed down.
With the exception of rap and hip-hop, you can argue that most of the other forms/genres of rock and pop music emerged at least in some prototypical form between the last half of the 50's and the 60's. Just looking at the decade of the 60's, you went from 50's style rock and roll, to early 60's do-wop, crooners like Pat Boone and Franki Valli, the whole British invasion, proto-hard rock and even proto-punk (e.g. the rawer Kinks and Yardbirds songs), acid rock, the birth of metal, etc.
You can also argue that musicians don't really create anything too new after they are past their late 20's, even though they can still be creative in an evolutionary sense.
By the same token, music listeners tend to like what they liked during their teens and twenties, and mostly want to hear more of the same from then on, or at least things that have a stylistic root in what they grew up with.
