This is just wrong. There wasn't anything comparable to Deng's reforms in 1980s USSR - indeed the Brezhnev-era folks and their immediate successors were pushing for more socialism, not less. Even Gorbachev didn't exactly start out as a pro-West social liberal!
The key difference was western companies were happy to (quietly) transfer expertise and technology to the Chinese. In the late 80s the international oil giants were helping the Chinese drill for oil while the Soviets were going (more) broke trying to drill theirs on their own, while Bush claimed “no bailout for Gorbachev”.
The west squandered a chance to implement a “Marshall plan” and make a strong ally out of the Soviets. Instead they chuckled at its collapse and danced on its grave, while rewarding the Chinese for choosing the path of authoritarianism, and here we are.
We definitely made this particular mistake much later on, in the 1990s; I'll give you that. But what you're saying wrt. the Brezhnev era requires some evidence if it is to be plausible. If perestrojka was Brezhnev's idea, it sure seems weird that it would only be implemented with Gorbachev.
So why weren't they?
Besides, after all the decades of drabness nobody really believed in some imminent glorious future as a justification anymore -- like they could still believe in the 20s and 30s...
The leadership and military of the USSR were no longer willing to violently suppress demonstrations in the 1980s.