I think that in 1968 music became angrier and more political. Silly over-paid musicians seemed to actually think they could change the world. Quite a bit of the music got ugly and tiresome. This trend was not universal, but it was long-lived.
Now, having said that, I have to add that the 70s and 80s are my principle musical era. I like progressive rock. I like the anger of the punks. I like the swing of 80s synth-pop. I would never say that "It was all downhill" after 1968.
But a lot of bands got harsh when the hippies turned into communists.
You are probably already aware of a documentary on the British sound called Rock Family Trees - The Mersey Sound Which shows the kids of the fifties in England could not afford good instruments or music lessons, so learned on their own, thus creating a new sound . Its worth watching if you're interested.
I tend to think 1968 - 1972 was the pinnacle for rock music and it's impact.
White Album. Beggars Banquet. Let it Bleed. Abbey Road. Sticky Fingers. Let it Be. Exile on Main Street. Paranoid. Master of Reality. Meddle. Electric Ladyland. Band of Gypsies.
Need I go on?
On the other hand, there’s the Grateful Dead...