By what point was the Soviet Union's fall inevitable?

[Orser67]

I'd say it wasn't really inevitable until the 1991 coup. But I would also point to 1964, when Brezhnev replaced Kruschev. Kruschev could have been the USSR's Deng Xiaoping (not as a precise analogue, but in general as an economic reformer who inspired future generations) but instead the empire stagnated under Brezhnev's conservative leadership.