As to any suggestion that there was nothing good in Rock after Sgt. Peppers I'd point out that Led Zeppelin wasn't even formed till two years after that album. Pink Floyd's entire career, except for one album featuring Sid Barrett, came after Sgt. Peppers as well.

Not to mention Dire Straits or Queen.

It should not be forgotten that R&R/Rock music were popular styles aimed at a mass market. A great deal of it was and still is utter dross, but among the dross, were a number of creative originals who produced music that will last and these were not confined to any period, though some periods seem to have been more productive than others.

That said, my wife recently bought the Dreamboats and Petticoats compilations which comprise pop music from the late 50's & early 60's, a period which is generally held to be one of bland pop music with a very high proportion of dross. Yet among the tracks featured are a surprisingly high number that I have fond memories of and enjoyed listening to again, largely, I suspect because they are from my teenage years.