The Soviet Union collapsed less than 5 years after Deng retired in 1989 and his economic model had only begun to bear fruit in the mid 80s. “For decades” is a wildly ignorant thing to say.
Chernobyl hastened the end. Not only was it an enormous ecological and economic disaster, that caused, as the article says, a loss of confidence in the central government, but it was a boost for the Ukrainian independence movement. And without Ukraine, there really is no Soviet Union, no empire. Just Russia and few satellite states.
What I’m saying is that Chernobyl was evidence of rot at the core. Really powerful evidence, but the rot was a fact, not a perception. If not Chernobyl, and I really wish it had been anything else, something else would have gone spectacularly off the rails because while you can hide many truths, incompetence and apathy both tend to reveal the truth of themselves. Corruption can conceal itself because it serves its own purposes. Concealing apathy is a fight against its own motives.