As others have noted the main problem for the Soviet Union was that the costs of paying off all the parties that make up a state (such as the civil service, the army, and ordinary people) were getting higher and higher, while the amount of money the Soviets were pulling in was shrinking (in large part due to the fact that oil was falling and continued to fall until the late 1990s). The Soviet Union can't go totalitarian, because the political/social costs were far too high, so there's a limit to how repressive they can get. There's also the fact that there were a number of reformists within the Soviet leadership, and as the situation gets worse they're going to have an easier time pushing for reforms (the costs of maintaining hardline policies are going to be higher than the perceived cost of reforms).
Ultimately, I would say that if the Soviet Union can survive until the late 1990s, when oil prices go up, they can probably ride that wave for a while (it's basically what Putin has been riding off of until recently). However I'm skeptical that the Soviets can hold out that long, and I'm even more skeptical that they don't eventually try reforms before then, which would lead to a situation similar to OTL just later.