I voted for better than 25% but less than 50%. I don't think that in the infinite multiverse, TLs with PODs after Khrushchev's removal from power wherein the USSR survives after 1991 are in the minority. However, it certainly is possible, but you must get rid of Brezhnev, and replace him with someone competent, preferably a reformist. That, assuming there are no Afghanistan-like military misadventures, probably would be enough to give the Soviet empire a 20-year lease on life.
Over 50%. I don't like to say it, but America at the time could easily could ahev lost the Cold War then. The USSR could easily surrvive, just remove Afghanistan, and reform the country.
Gorbi was a competent and good leader and he couldn't do that