Firstly - Medvedev is supposed to be the mouth of Sauron - he shouts out madness to make Putin look good. Putin is not going to use nukes for loss of what he has temporarily conquered.
Secondly, it isn’t about saving Zele the clown, but rather preventing Russia from getting too confident to then invade a NATO country
I don’t think Russia will invade any NATO country with Putin in leadership. Ukraine is not a NATO country and has very large common land border with Russia. I can understand why Russia/Putin does not want Ukraine joining NATO.