I believe that it would have collapsed...maybe not in 1989, but sooner or later, yes...it would have fallen in on itself like a house of cards.
I've read quite a bit of info that has come out of the former Soviet Union since the early 1990's. While in my mind there's no question that Ronald Wilson Reagan was the guy that succeeded in killing off the USSR, the Soviet system was so corrupt, poorly led, and managed that it was only a matter of time.
I started soldiering during Reagan's first term. I knew that the Soviet beast was immense, and that defending against its oppression would be my chosen profession. A few years later, I found out that the USSR was a paper tiger...and that they were one hell of a lot more scared of us, and really worried that we would attack them in mass.
I guess it's not that hard to believe. During the Great Patriotic War, the USSR lost 20 million people against the German Army (assisted by the Finns, Bulgars, Romanians, Italians, Croats, etc.) Think about that for a moment. The leadership felt they had to have the biggest army in the world - so they never really drew down from WWII. They had a couple of reasons for this; one being the West/NATO, and the other being China. But it was too big, and too unwieldy. They couldn't supply their soldiers in the field, they couldn't train year-round, since they had to send their soldiers to cut wheat, and they couldn't afford to train their soldiers properly without professionals in their ranks.
The last decent leader (from their point of view) was Nikita Khrushchev; Brezhnev was a broken man afflicted with severe Alzheimer's, Andropov was more concerned about getting rid of dissidents, and Chernenko lasted, what, a year? The nation depending on external sources for FOOD, and couldn't manage to push forward any decent programs outside of the major cities. For the peasants that survived the purges of the 1920's and 30's, nothing really changed for them.
Of course, Stalin didn't really help things get better, as it's a toss-up on who killed more Russians during WWII - him or the entire German Army. He let engineers do earth-moving projects in the early 1950's with nukes, for Pete’s sake!
A lot of knowledgeable people feel that Russia could be an economic powerhouse, provided it gets a grasp on its natural resources and learns how to take care of its citizens. Perhaps a few light bulbs will finally go on over there, and they'll join the 20th century, and possibly the 21st.