As much as I would like to agree with this, who the hell really knows? This whole campaign in Ukraine has shown them targeting civilian infrastructure as much, if not more, than military targets. At some point in time, I'd imagine most of their nukes were pointed at military installations around the world, but at least some of them are likely targeted at London, DC, NY, Paris, etc. Gaming this out, if Putin wanted to put himself in the strongest position (not strong, but strongest), as you did in this same post when you said:
"The point isn't that we wouldn't leave a few missiles behind (though the goal is that we get 100%). It's that after that initial strike, we have an opportunity to open a line o communication and say, "look--that was a real bad hit. But we didn't hit any of your cities. So everything's fine. But we still have 1,200 warheads; you have just a handful. So let's call a ceasefire and put this thing to bed."
If he's able to get off even one of the remaining missiles at a major western city, your idea doesn't go so well, and nukes start flying everywhere. I wouldn't be surprised to learn that a lot more of them have been re-targeted at civilian populations recently.