If Putin uses a nuke in Ukraine, NATO will respond by decimating the Russian invasion force in Ukraine (probably excluding Crimea) with conventional air power. That should be seen as de-escalating, since 1) only a nuclear response can really be an escalation to a nuclear provocation, and 2) Russia’s pre-war border is not violated. It will allow the Ukrainians to take back their land to pre-war boundaries (“Vietnam”). Putin knows this (it’s probably been “explained” to him by Western leaders), so the likelihood he would choose that path is small. Even if he does, I think the likelihood he would opt for further nuclear escalation with a first strike beyond Ukraine is much smaller than you claim. The West would never opt for a nuclear first strike, because of their clear conventional advantage.
In other words, both the kaboom and the KABOOM must be initiated by Putin and have no upsides for him. That’s two huge hurdles to Armageddon, and I find your probability estimates overly pessimistic.
If Putin uses a nuke in Ukraine, NATO will respond by decimating the Russian invasion force in Ukraine (probably excluding Crimea) with conventional air power. That should be seen as de-escalating, since 1) only a nuclear response can really be an escalation to a nuclear provocation, and 2) Russia’s pre-war border is not violated. It will allow the Ukrainians to take back their land to pre-war boundaries (“Vietnam”). Putin knows this (it’s probably been “explained” to him by Western leaders), so the likelihood he would choose that path is small. Even if he does, I think the likelihood he would opt for further nuclear escalation with a first strike beyond Ukraine is much smaller than you claim. The West would never opt for a nuclear first strike, because of their clear conventional advantage.
In other words, both the kaboom and the KABOOM must be initiated by Putin and have no upsides for him. That’s two huge hurdles to Armageddon, and I find your probability estimates overly pessimistic.