I'd extend your timeline up to the dawn of the Reagan Era. New York in the 1970s was a party that will not happen again but am I ever glad I got there in time for it.
Yeah, 1980 was def the end of it all, Sharky, especially once the plague hit.
For me, though, a lot of the 'spark' had gone especially by the mid 70s. i2 mentioned Manson et al but there was so much else like The Weather Underground, and the kidnapping of Patty Hearst, that (IMO intentionally) fucked up the progress 'the left' had been making. Essentially the rest of the country began to perceive the counter-culture as domestic terrorists, a genuine threat to Constitutional government. It is near this time the word "liberal" became sullied in the US vernacular:
"Freaks are Revolutionaries, and Revolutionaries are Freaks."
"Mom, Dad?… I'm with a combat unit that's armed with automatic weapons…"
"Death to the fascist insect that prays upon the life of the people!!"
This was all TOTALLY WTF