Khrushchev tried to reform the system and was overthrown for his efforts in a party coup by those vested interests that would have had the most to lose. Gorbachev must have been very aware that a party coup was possible when he began his tepid reform efforts. His reforms served mainly to weaken the state apparatus, which in turn to midwife the sudden collapse of authority. The coup did come in fact; fortunately for us it was amateurish and far too late: it is now almost forgotten. The difference lay in that during the 30ish years between Khrushchev and Gorbachev, the people lost all hope and the vested interests had become complacent and effete.
Brezhnev was a party hack and pleasure seeker who was largely a caretaker and weak leader during his Chairmanship; empire building within the Soviet system ran amok during his tenure. Resources were looted and sold to the West for cash, specifically U.S. Dollars. The USSR didn’t make any product Westerners wanted, but they did have large amounts of Oil, Gas, Gold, Gems, rare metals etc. They even built a natural gas pipeline to Western Europe at their expense to sell cheap gas. So, the USSR became an exporter of raw materials and an importer of finished goods and food. By an economic definition they would almost be a colony. The finished goods went either to the well connected or the military and in both cases created a junkie like dependence. The productive effort they could make went into largely into military equipment, which if unused, is wasteful and counterproductive.
In 1980, the Soviets put about 40% of GNP toward their military, still fully mobilized 35 years after the defeat of Nazi Germany. In 1980 we put about 3.4% of our GNP toward our military, our NATO allies somewhat less. I am very skeptical that the increase under Reagan from 3.4% to 3.8% of our economy devoted to our military buried the USSR. More likely is a multi part proposition. The USSR was already exceeding the long term carrying capacity of its economy for the effort devoted toward the military: therefore they either had to reduce the military burden or expand the base upon which the burden rested, i.e. conquer and annex Western Europe. The problem with reducing the burden was that while that would be good for the nation, it would be bad for Soviet vested interests. The problem with conquering Western Europe is that a/ Western Europe could be wrecked in the process, which is a severe problem due to their dependence on Western finished goods and food, and b/ no U.S. Government would let that happen without a fight and c/ the fear that the Eastern bloc satellite countries or non Slavic parts of the USSR revolt, might rebel or simply tie down troops. The extended problem then with fighting is that the USSR could not add any additional military effort beyond the current efforts, yet were the USA to devote 40% of GNP or the roughly 28% of GNP we used during WW2 we could expand our forces anywhere from 7 to 11 times their current size. So the absolute maximum potential of the U.S. economy directed toward the military was far too great to risk unless the Soviet leadership were desperate. Whether or not another American infantry brigade or missile battery was sent to, or taken from, the European Theatre during that era had only miniscule significance. The Soviet leadership never found a viable strategy for victory, acted in accordance with the belief that the West would initiate conflict, and would not make any hard decisions: then dithered until finally, it was out of their hands.