...just a thought I had. When the Chinese changed horses and cracked down hard on the protestors, the world looked away. Times were changing, but it could be said that perhaps people didn't want them to change as quickly as they were.
What if the Soviet military had cracked down hard on the protests that would be the undoing of the country? There were still troops (lots) loyal to the state even after it ceased to exist. One could even say that for many governments a continuing USSR would be preferable to an unknown entity/entities popping up in it's aftermath. Any number of nightmare scenarios (loose nuclear weapons, rogue commanders) could be imagined by the administrations of West Germany, France, Britain, and the US and perhaps the best option in the case of a crackdown would be to shake one fist in righteous indignation while using the other hand to wipe their brows at the lucky break the Soviet military had given them.
This isn't just about the immediate effects of a continuing USSR, of course. When the Tienanmen Square Massacre happened, the world very conspicuously began it's first foray into a new world of moral gray areas: one where we hedged our bets against our consciences and hoped we wouldn't pile up too many regrets.
But how many regrets would there be after a Soviet crackdown in 1991 or 1989?