Altamont closed the 60s. The only change that show ushered in was the demise of Hell’s Angels as concert security (it’s still disputed if the Angels were primary security).
Bill Graham and Jimi Hendrix closed the 60s.
At the Fillmore (East?) Jimi played a New Year’s Eve show and a lengthy version of Auld Ange Syne.
Bill Graham announces it is a new year, a whole new decade.
And the 60s were gone.
Those were 2 icons of the 2nd half of the 60s so that was a definitive statement.