The very point is they didn't give up the buffer zone 'voluntarily'. They just didn't have a choice. The Soviet Union itself was collapsing under the fight of national, social and economic problems.
Everyone has a theory of Soviet collapse that fits his preconceptions. Some people in the conservative movement want to give Ronald Reagan all the credit. Others want to say it was simply the inevitable outcome of having a socialist economy.
Who knows? After every great historic event, there are those who claim it was inevitable, that powerful social forces made what happened, inevitable.
Although I definitely believe in the power of 'social forces', I'm also a big believer in historical accident, and the historical accident here is the personality of Mikhail Gorbachev, one of the great figures of the 20th Century.
I don't believe the Soviet leadership had 'no choice'. They had plenty of precidents that would justify repression, if necessary, as in E. Germany in 1953, Hungary in 1956, Czechoslovakia in 1968.
The Chinese Communist Party has successfully held on to power, while ridding itself of the socialist millstone around its neck. North Korea and Cuba haven't even done that.
Of course, it's impossible to come to a 100% certain conclusion when we play 'what-if' with 'alternative histories'. But I believe that if the idiot new Kaiser had retained wise old Bismarck as Chancellor of the German Empire, we would have avoided World War I. If Lenin had not been able to return to Russia in 1917, we wouldn't have had the Bolshevik Revolution. Perhaps other catastrophes would have ensued, but not those particular ones.
I lived in the Soviet Union for a few months in 1985, in Kharkov. It was not my sense that there was widespread, active discontent. It was certainly true that the intelligentsia knew that their system couldn't keep up with the modern world.
Microcomputers were just coming in at that time, and I took my BBC Micro with me [roughly equivalent to the Apple II, but better ], and gave some talks on "Microcomputers and Education" in Kharkov, Academgoroduk (in Siberia) and in Tallin, Estonia. Soviet computer people were very interested in my little micro, and especially in the fact that it had cost me less than one month's salary.
The Moscow bureaucrats had supplied them with mini-computers (a knock-off of the PDP-11, I believe) but had neglected to equip it with software -- so every university had to write its own compilers. Similarly, they had purchased microcomputers, but neglected to make them compatible with the Cyrillic alphabet.
When we returned home, I predicted to my then-wife that we might see the end of the Soviet Union within our lifetimes -- despite the fact that most academic 'expert opinion' in the West was that the Soviet Union was rock-solid and would endure indefinitely. This was not because people were starving, which they were not. It was because the system had lost the support of its intelligentsia. Rather like the US today.