His chart [1] indicates where the 1975 date came from. He distinguishes between 1960s "counterculture" and later "subcultures". That's somewhat artificial. The 1920s also had subcultures - the Jazz Age, flappers, etc. Prior to the 1920s, there wasn't enough mass disposable income for such frippery. The 1930s (depression) and 1940s (WWII) sucked so bad that there was no counterculture, subculture, or, indeed, much culture.
The more interesting question is "why did it stop"? Music stopped being an agent of rebellion around the time punk died. Now, it's just "content". When it became easy to distribute music, everybody started doing it. At peak, there were several million bands on Myspace. Subcultures became more fragmented and tinier. Once one could find kindred souls with similar narrow interests on line, the need to change to fit in was much reduced.
What the author calls "fluidity" is the endgame of that - subcultures have no lasting power in a world of tweets and Instagram.
Were any of these a creative force? Outside of music, the main legacy of the hippie movement is that everyone wears jeans. The values didn't stick. Much of the 1960s counterculture was just self-indulgence dressed up with pop philosophy. That's why hippies transitioned so smoothly to yuppies. The legacy of punk is industrial interior design, for which the endgame turned out to be open plan offices with exposed brick walls and ceilings with visible pipes and conduit.