The downfall is right there when they tried to be clever and intellectual but sounded quite corny most of the time. And Abbey is just dull, with the exception of brilliant "Something" not written by Lennon/McCarthy.
George Martin´s role in production is very important, he truly was the fifth Beatle and without him the end result would have been different. "Revolver" and "Rubber Soul" have real magic that they lost when they became "serious" musically. Innocent and very happy (pop) music, simple tunes like "Girl" are their finest hour. Of course, later they had their moments like Back in the USSR, they finest rocker. Heh, Sovjet Union. Very funny tune for us, our neighbor you see.
Lennon could only dream of songs like "A Whiter Shade of Pale" not to mention "A Salty Dog". Btw, the perfect pop music perfectly produced by class came from Sweden a few years later ... tunes like Ring Ring and SOS. And from Holland, Never Mary a Railroad Man, not to mention Venus.
In late 60´s people such as Gary Brooker & Co and Jimi Hendrix & Co created the very first attempts to serious and important rock music ... "Electric Ladyland" from 1968 is light years away from harmless pop music. Hendrix was/is a genius and way ahead of most pop musicians. These guys created new genres, new art. Symphonic Prog and Psychedelic Rock and the world was never the same.
I read somewhere that the "mighty" Beatles stopped to perform for audiences already in 1966 because they were tired of screaming fans, in the middle of Beatlemania ? Really. How stupid is that. Quite an insult to the fans. The most overrated music in history. I like their 1964-66 period, though. But never my cup of tea really.
Btw, for a pop band I´ve always preferred The Hollies because their songs were simple about love (girls) and their songwriting is very good. When pop music becomes too serious it kinda looses its magic, as a simple pop music.
And Allan Clarke had a gorgeous voice, perfect for pop melodies. And great harmonies from Graham Nash, but that is another story.