Correlation != causation. For one, all kinds of Eastern European regimes collapsed before USSR -- which helped a great deal.
Second, it was already not in very good shape from the inside...
Third, China did the same closer capitalist cooperation at the same more or less time as USSR, and is stronger than ever, as opposed to collapsed...
My point was: that may have been (in part) thanks to the Western policy. If you're confrontational, you'll strengthen inner unity, and if you isolate a country, you aren't likely to convince the population that a different system of governance is possible.
The leadership and military of the USSR were no longer willing to violently suppress demonstrations in the 1980s.
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...
How so? It appears to have worked on the Soviet Union, and very few people accurately predicted its fall.
Increased living standards will surely bring change, any likely already have - China is a different place now than it was under Mao, isn't it?
There's also a cultural aspect, of course. "Change" usually implies change to become more like a Western society, but that's not necessarily what the Chinese want.