If Mao continued to push his continual revolution strategy - a thing also deemed attractive by many other tyrants as that constant state of revolution made killing off your political rivals that much easier - then it might well have driven China into anarchy. Even today China has some horrendous internal divisions that the CCP works mightily to keep from splitting to widely open. Under sustained failures due to Mao's evil, those divisions could become gaping chasms.
Sure, each faction would attempt to lay claim to being the "true" Communist government and the ensuing mess would be horrendous indeed. You might then see the Soviets attempt to intervene "in the name of the workers and the Revolution in the People's Republic!" That, in turn, would serve to unify at least some of the factions as repelling the foreign invaders would be a higher priority than squabbling over which faction was more Communist than the other. Other factions however, would seize upon the Soviet involvement as the means to "save the Revolution" and use it to strengthen their own position.
I don't know if the Nationalists in Taiwan could much capitalize on things by attempting to come ashore on the Mainland and "take China back for the Chinese!"
At the least it'd be a right sorry mess. Depending on the timing though, it might also serve to choke off the flow of Soviet weapons shipments to the North Vietnamese. And that would lead to greater US / ARVN successes. Perhaps even enabling South Vietnam to survive as a non-Communist nation. If so, then there'd be no Khmer Rouge either. So, no Killing Fields.