Then they never, ever would have been able to sustain the Cold War. This would have been especially true during the Stalinist reign of terror. Masses of people would have left to the West, to the point where it might have been Western countries putting up obstacles to immigration, but those who could would get out. East Germany might have lost most of it's population, as the FRG was accepting anyone who managed to cross over. The same is true for North Korea, assuming history goes much the same way, assuming the Cold War doesn't end before it falls behind South Korea (the North started out as having the better standard of living), and the majority of Soviet Jews would have ended up in Israel. The emigration would create a net effect of more emigration, as word got back of the better living standards and political freedoms the West had to offer.
While except for East Germany and North Korea, I don't think any of the Communist states would have experienced a demographically devastating drop in population, mainly because other Communist states weren't built from divided nations with a here is going to be a
massive brain drain to the West, as educated and qualified people able to get visas leave in droves. This is further going to depress the quality of life, as things like healthcare, education, and infrastructure all suffer due to a lack of doctors, professors, and engineers, but this will also critically impair the Soviets' ability to keep up with the West in producing new military technologies, and they'll fall farther and farther behind. The Soviets might try to offer all kinds of incentives to stay, until they've essentially create a bourgeoisie and defeated the whole purpose of Communism, but if the West is smart, it'll offer it's own incentives to drain the Soviet Union of more brain power. The Soviets could stem the flow through certain incentives and strings attached to free education, but barring drastic action (IE "if you get a state-funded education you must stay for 20 years"), they couldn't stop it short of closing the borders.
The failure of Communism will be painfully obvious for all to see, as people emigrate in droves to the capitalist West, with no discernible flow the other way. The Eastern bloc will fall further and further behind unless they take harsh measures to stem the tide.
Brezhnev was talking to his Minister for Internal Affairs.
"Comrade Nikolai," he said, "How many Jews do you think there are in our glorious union?"
"A little over 2 million, Comrade General Secretary."
"And how many of them do you think would leave if we let them?"
"Perhaps 3 million, Comrade General Secretary."
I think it was originally "perhaps 20 million, Comrade General Secretary".
Anyway, in another joke, Leonid Brezhnev is talking with Golda Meir, and keeps asking her out on a date, but she refuses. Finally, she says "I'll go out with you when you agree to let everyone who wants to leave the Soviet Union out. Brezhnev smiles: "Ah, I
knew you wanted to be alone with me.