This would be the end of Russia really fast, Putin included. I don't think they are stupid or suicidal on that level. And they have threatened to go nuclear for years now, there's little reason to believe that this time is any different. And ultimately it also doesn't change anything, we can't let Russia bully us with their nuclear threats. Because there will never be an end and if they could conquer the world with nuclear threats they would do so. So let them prepare their missle and have nuclear drills. And send more long range missiles (looking at you Taurus).