If anything the USSR economic policy was far more sustainable than what they have now. They had _manufacturing capacity_ and were almost entirely self sufficient. That is not the case today. And the stuff they import, they have to pay Euros and Dollars for. Ruble has been pretty much in free fall for more than a week now. Their population today (unlike back then) is not ready for a drastically reduced quality of life.
Source: Russian-American, lived through perestroika and the "wild 90s", still have family there.
Oh dear. An unstable Russia, a confident and resurgent China with something to prove, a United States distracted with its own internal problems... are we going to see World War III?
Not really, no. You can thank MAD for that. There's nothing to gain and everything to lose. Other countries understand the threat of proliferation and will swoop in at the last minute to make sure the weapons stay in the hands of someone sane, just like they did in the 90s.
Source: Russian-American, lived through perestroika and the "wild 90s", still have family there.