In the fall of USSR, no one was "afraid to shoot". No one was motivated to do so. Or do anything for the completely failed system that no longer worked for anyone, inside or outside it. By 1991, no one cared for good ol' Union. That's the only reason it died so peacefully in its sleep with virtually no violence - it was so rotten, no one could care for its survival.
Let's not forget how much violence there'd already been
And indeed that is how the Shah fell in Iran - the regime was so rotten that even the military elite no longer saw it as worth defending. Will the current regime face the same eventually?