The moment putin tries to do nuclear attack, he’s dead. Zelensky said it well, that putin spent most of the invasion in some bunker, before that he had this thing with ridiculously long tables being super paranoid about covid. He’s clearly not planning to die.

Also I think it helps to imagine the details. How would the nuclear attack look like? How would it prevent Crimea “falling”? What would be the breaking point? It’s not like there would be some clear change from Ukraine not touching Crimea to full on liberation. Ukraine is bombing military targets in Crimea even now. They are even conducting operations on Crimean soil right now. They hit the Kerch bridge multiple times already. Why press the button after 4th or 7th bridge hit and not after first?

Also it’s not like he can’t sell this defeat as victory. The way things are going there’s probably a lot of people in russia prepared to pretend that there was glorious victory after heroic attack towards russian border. There will be interviews in the Moscow streets with people praising the successful special military operation, because they know that if it continued few more months, there would be conscription in Moscow.