I never smoked dope, and I loved sixties music. What happened was the people who made the best music had a certain amount of music inside them. My guess is that too much dope ruined their creative abilities. Few of the big stars made good new music into the seventies.
The Beatles for example. After the sixties, McCartney's new music was mostly pathetic, maudlin crap. Ditto for John Lennon.
George Harrison made the best post-Beatles music (though John Lennon's
Plastic Ono Band and
Walls & Bridges were pretty damn good too . . . )

Bob Dylan's new music in the seventies was largely mediocre garbage as well.
Drugs probably ruined more good musicians than made good music.
Dylan was never going to make anything as good as
Another Side of Bob Dylan,
Bringing It All Back Home, and
Highway 61 Revisited again. (Who would?) But he got as close as he'd ever get with . . .

I think sometimes of the performers who didn't make the cut of the
Woodstock film, for assorted reasons---the Band, the last solid edition of the Butterfield Blues Band, Creedence Clearwater Revival, Mountain, Sweetwater.
And, some of those who were invited to play Woodstock but didn't appear there for various reasons: Jeff Beck (his group broke up before the festival), Bob Dylan (invited but signed to play the Isle of Wight instead), Simon & Garfunkel (declined because they were working on an album), Led Zeppelin (declined when their manager complained they'd be just another band on the bill, despite them playing the Atlanta Pop Festival earlier that summer), Tommy James and the Shondells (they were told only that "some pig farmer" was putting on a show, not realising a) it was dairy farmer Max Yasgur lending his farm to the festival, and b) that it was going to be Woodstock: "We realised what we missed a couple of days later!"), the Moody Blues (had to decline because they were booked for concerts in Paris the same weekend), Joni Mitchell (her manager feared it would cause her to miss an appearance on
The Dick Cavett Show), Procol Harum (their then-guitarist Robin Trower's wife was due to deliver their first child that weekend, after a long concert tour), the Rascals (like Simon & Garfunel, they, too, were recording a new album), the Doors (they were led to believe Woodstock would only be a second-rate imitation of the 1967 Monterey Pop Festival and declined, to their regret), and Iron Butterfly. (The Butterfly were stuck at La Guardia Airport and wired promoters demanding they send helicopters to the airport, fly the band in to the grounds, fly them out immediately after their set, and the band was promptly disinvited even though they were scheduled to play that Sunday.)