In the counterfactual where the Soviets conquered Europe, I could easily see the USSR surviving to today... No liberal democracies on their borders to compare with, no need for a huge military on the western border, and increased resources would go a long way.

It’d still crumble under its own weight due to corruption and lack of mid-level society bonding factors like corporations, unions, gatherings, etc. China managed to navigate this mess somewhat successfully until it was technically possible to implement social engineering on a scale previously unheard of with their great firewall and social score. Compute power enables their selective repression system.

What makes you think the USSR would have been more successful had it been stretched even more thinly, across a much larger land area and population, where the vast majority of those people would hate their occupation overlords?