In fact, I think part of the genius of the Beatles was that they were among the first bands that recognized how hallucinogenics could open them up creatively...and then quickly realized how much of a dead-end that could lead to. The White Album is about as un-psychedelic as a late-60s record can be. If they had tried to make "Sgt Pepper II," it probably would have been a disaster and their legacy would have been hurt severely.
Dylan recognized this too. Blonde on Blonde is arguably the greatest album in rock history, but if he had continued writing and recording that way (and somehow managed to survive the drug use tied to it), he would have been a complete self parody by the time he turned 28.
Smile is probably the all-time great example of what happens when an artist goes too far down one particular rabbit hole. The Beach Boys were basically a nostalgia act by 1970. (Though I actually like some of the early 70s stuff. And, in fairness, Brian Wilson was probably the last person on earth who needed to experiment with drugs).
I think, generally speaking, drugs are given too much credit for the creativity of the late 60s music scene. It played a role, obviously, but it's not like mediocre artists smoked pot and were suddenly the Gershwin brothers.
It seems like you need to hit that sweet spot with drugs. I love Exile on Main Street, and I don't think Exile sounds like Exile if Keef is not a junkie at the time. But that also was pretty much it for Keith as a songwriter.