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

[Middle-aged Europe]

Sometime during the Brezhnev era, so the early to mid 70s are a good guess.

Gorbachev's reforms obviously came to late, so it must be earlier than 1985. You can also tell that your empire is in decline when you start to prop up geriatric leaders like Andropov and Chernenko who kick the bucket shortly after coming to power.

Invading Afghanistan was also a major mistake, so I'd say 1979 is the latest possible "point of no return". But it was probably some years earlier than that. The Soviet economy turned stagnant around 1970-75, so that's the period I would settle with.