The massive conscript army (and border troops and internal troops, etc) was not just a military precaution
Regime stabilization played a part, but not only just in terms of repression. Rather, as a social institution that carried out political education, acculturation for people often from underdeveloped peasant/transient labour backgrounds, as well as manpower for the more menial aspects of the agricultural and construction sectors, this was not going to be done away with just because militarily there was no urgent need for it. If I could compare it to a modern example, Israel does not need a large conscript army, it's major threats are not Egyptian Tank Divisions trying to seize Ashkelon, but rather are more asymmetric or long range in nature - but Israeli society sees enormous social utility in a conscript army to the point where it's structure is embedded in social life and economic expectations. The Soviet Army is similar in this regard. The Soviet State, and its satellites, existed in an almost permanent state of mobilization, with a state security fetishization that was extreme even for 20th century standards
Even with the massively overgrown military sector, however, it's not as if the Soviet economy couldn't make tangible economic gains from this, as the Americans did from the Sunbelt States Defense R&D sector that sprang up in the 60s and 70s. Rather, the Soviet economic problem came in that their policy of investing constantly in new capital equipment for the purposes of, well, making new capital equipment, made it an economy where there was little capture of value add from either production or consumption, a d this had marginal returns over time. All of the issues with corruption, distribution failures, poor standard of living, bad and inaccurate data for Gosplan, these made bad things worse, but they weren't the massive albatross crushing the economy once the returns of postwar recovery and imperial looting from Eastern Europe were exhausted. The Kosygin reforms may have eventually found a way if pursued adequately to get more out of the inefficient industrial sector, but not enough to fix the core problem.
I tend to concur with Stephen Kotkin in Armageddon Averted when he notes that the 70s allowed the Soviet Union to ignore this massive problem because energy prices covered over a lot of revenue shortfall gaps and therefore they were shocked when they shouldn't have been when that came to a halt. Most countries had a rather nasty adjustment period in the 70s-80s from an industrial mixed market economy to a services based more market dominant economy. The Soviets didn't. Their satellites papered over this problem by just getting extremely indebted.
