How is that an existential threat? No massive waves of nukes were fired out of their silos, no bombers entering Soviet airspace, no invasion crossed the borders. Impending threats yes, but not like they had their territory violated.

(EDIT: Misread it. Regardless, the USSR perceived an existential threat by NATO, and responded by threatening existential defeat for NATO by readying their nuclear weapons for launch. They didn't want to actually fire them until they have confirmation of nuclear war, and this is behaving in a manner consistent with MAD.)

Even if the USSR interpreted the malfunctioning early warning systems as an actual nuclear attack, and thus existential defeat is imminent, and behaved in your scenario by Kessler Syndroming GEO as a "symbol of their willingness to go nuclear" (pretend for a moment that Kessler Syndroming GEO is actually viable in the 80s), only find out that this was due to technical glitches and aggressive drills, what they would have done was escalate the scenario by destroying NATO assets, increasing the likelihood of leading to an actual nuclear war, which will be the lose condition for everyone.

What the USSR did in reality in response to believing that NATO was escalating to a first strike nuclear launch was more in line with the usual MAD response. They readied their nuclear arsenal (knowing that NATO would realize what they are doing), and prepared to fire back in the event of a nuclear strike.

Again, no one wants to activate MAD. It's purpose is that once a nation acknowledges existential defeat, it has the option to bring everyone else to hell with it in the hope that the threat of doing so is enough to avoid existential defeat in the first place.

You are either facing an existential threat, or you aren't. MAD isn't about winning, it's about making everyone, including yourself, lose.

If you aren't facing an existential threat, you don't destroy something valuable to neutral and allied nations permanently just to get some temporary advantage or "to show you are serious". Destroying common resources permanently only pisses off others unrelated to the conflict and is guaranteed to escalate the conflict and push it closer towards an actual existential threat, making it more likely for everyone to all lose under MAD instead of simply threatening to push the button to get the enemy to back off.

If you are facing an existential threat, MAD with nukes is the best option. You don't need a "symbol of your willingness to go nuclear". You just show off your nukes, and then show a red button and threaten to press it. No half-steps, especially permanent half-steps like Kessler Syndrome GEO, because that just pisses people off and makes it more likely that you have to press the button, which you don't want to anyways because you will still die.

It's Mutually Assured Destruction lite. Sure, Kessler Syndrome in GEO is less instantly lethal, but the point about MAD isn't for half-measures. You go big or you go home.

If you suffer an existential threat, you don't merely inconvenince future generations by making weather satellites more inconvenient. You threaten to kill everyone in nuclear hellfire to prevent existential threats.

What you're suggesting isn't something trivially done like magically temporarily stop all aircraft flying for a strategic leveling. It's more akin to permanently destroying Geostationary orbit functionally forever.

This is more like the Matrix's "Let's blot out the Sun forever to get an advantage against the Machines" level decision. Maybe useful for this one battle, but then even if you win, you are worse off than just flat out losing conventionally.