Actually them going to nukes over Ukraine would be the canary in the coal mine.
It would mean that nukes were already an inevitable outcome on this as they wouldn’t stop after it and we know it.
They would push, get rewarded for it, then push again after they recovered as they know none will stop them and so on.
At which point it would be better for the nukes to come out now than when they are in an even more favorable position from getting away with that threat before and getting rewarded for it.