Hold on, didn't China to minimal deterrence? The whole reason I suggested this was because Maoist China ended up following that strategy despite the issues it had.
With that in mind, my idea is that a reformer comes to power in the 1970s somehow, and decides to take advantage of the US's cultural issues at the time to transition the Soviet Union into some sort of,"Peace Communism."
The Chinese made a virtue of necessity. They couldn't possibly match both Soviet and US build rates, they couldn't possibly develop a first strike capability against either, let alone both. So they built just enough to put either on notice it wouldn't get away with a unilateral strike without paying a steep price and let it go at that.
I don't know about you, but I was around in the US during the '70s. I was a minor to be sure, but as a nerd and a military brat I paid attention to this stuff. As a young adult in the '80s I became a belated hippie who would totally agree that the US could and should build down and back off, but the cultural setting I lived in in the '70s was a lot more hawkish than your impression of the decade allows for! To be sure the idea of detente was in the air, and the hawks often gave the impression they were carrying on a lonely Cassandra crusade against a bunch of half-baked peaceniks who would just open the doors to the first Soviet paratroops who came wafting down. But that wasn't really the case! I didn't live in places like New York City to be sure. But the places I did live in, mostly near US air bases in the Deep South, were skeptical of treaties with the Russians. Even living in Southern California as I did one year my Dad was on duty overseas, in South Korea, the parts of Los Angeles I lived in were hardly bastions of nuclear disarmament. Aside from my Dad I had an uncle in the Strategic Air Command; I had other relatives working for various big defense contractors. And while the majority of people I met in the streets of the parts of LA County bordering on Orange County in 1975 were not so deeply inducted into the infamous military-industrial complex, I guess, no one was thrilled by losing in Vietnam, nor was there overwhelming sentiment for winding down. I certainly never got the impression from the national media I was paying more and more attention to that it was a consensus we had too much military; such views were heard but they were always shown as marginal and "irresponsible."
Of course we actually did have the SALT Treaties, but the ink was hardly dry on them before quite a lot of American politicians with grim-faced generals standing behind them were ranting on prime-time news programs about the foolishness, indeed wickedness, of signing them.
Looking past the media frenzy and other fogs, it's fair to say the US governing establishment was coming around to the idea of living with the Soviets for the long term, and making and keeping treaties with them. But don't underestimate the forces of social inertia and economic interest I have mentioned, I saw them at work. I do believe the Reagan administration owed a lot to backlash against the very idea of accommodation, a backlash its hawkish wing (which in the early years, appeared equivalent to the whole thing) personified.
The point is, a Soviet onlooker, even one advised by very sophisticated analysis, could hardly get the impression the Americans all unanimously were for peace at any price, or in some cases peace at all. They could hope the more extreme, frothing-at-the-mouth rhetoric was just that, or the sincerely held but marginal and irrelevant crazed views of a lunatic fringe, but calls for a revival of the crusade against Godless Communism were certainly being made--and published in various media, all across the country. How much confidence could Brezhnev, or some alternative honcho of much firmer moral fiber, place in the basic goodwill of the USA? Our hawks gave points to their hawks; similar economic incentives (not optimal for the nation as whole, but vital to the large numbers of people already beholden to already bloated military logistics) operated on both sides. When the Soviets did develop capabilities to match the new ones we were constantly developing right through the '70s, that was further ammunition for American hawks to denounce the insincerity of Soviet intentions, and it went another round. In retrospect it's perfectly clear the Russians were always playing catch-up, but at the time I would read one alarmist article after another about how the Russians were stealing a march on us and we had to rally and catch up to
them.
The arms race was alive and well on both sides, and the American side gave no clear, unambiguous sign that we were seriously going to let up. Too much pork, too deep an emotional investment in the idea of American supremacy as the sole guarantee of our survival, for a dovish move to have enough credibility for the Kremlin to bet their lives on it.
Reagan himself may have been the wild card that eventually let Gorbachev bet on it. He'd seen
The Day After, the mid-80s dramatization of what a nuclear war would actually be like in the USA, and it shook him. He suddenly wanted assurance we weren't headed recklessly for an unnecessary Armageddon and he wasn't getting enough of it from his own hawks; that's when he suddenly turned to serious negotiations with the Soviets. But clearly the mid-to-late 80s was too late for the USSR.
No, if the Soviets were going to go over to a doctrine of minimal deterrence in the '70s, it could not be justified by saying, "look, the capitalists are calling uncle on this mad buildup, they are asking us for a mutual cool-off!" They'd have to boldly go ahead and take the chance that yesterday's adequate minimum deterrent would tomorrow be vulnerable to some expensive but effective development in the West. Like MIRVs for instance; the idea of putting lots of warheads in one missile was developed in the West first, partially justified of course by the "inevitability" the Soviets would soon have it (if not already) and they upset the balance of power, because now one missile could strike at many silos or bases; vice versa the cost of losing one MIRVed missile was that much higher. They'd have to keep on top of such Western developments, in the absence of credible treaties (and OTL, American politicians never tired of saying a treaty with the Kremlin was worthless, so how much trust could the Russians place in our assurances?) they'd have to counter new American capabilities to decimate their minimal deterrent, probably by multiplying it. Every time they did, it would be touted as further evidence of sinister Russian intentions.
Actually even at their feverish peak, the US and Soviet strategic nuclear arsenals were only a small fraction of the total cost of each year's military budget. The big savings would come in if both sides could agree to build down their non-nuclear, non-strategic forces. By the same token of their sheer economic costs, the political inertia alone, not to mention that both sides foresaw a real and ongoing need to actually use all this firepower, was against such a savings.
I do think the Russians, and still more the USA, could have afforded to take the risks involved in unilaterally downsizing and redefining the purpose of their massive strategic nuclear forces in the 1970s. Unfortunately neither side saw the compelling need--the Brezhnev years, though obviously in retrospect built on sand, were years when ordinary Soviet citizens saw themselves as being better off than ever before and that the regime's long-uttered promises of progress and security might not be so empty after all. Or so I've read anyway.