If Putin dies, a successor might (secretly) align with Ukraine. Something like "If you attack XX, I will remove my troops from ZZ, if you don't attack me. I will be focusing on a bread & butter campaign if I run Russia. Take care of my rivals in the meantime, mm'kay?"
Mind, I would prefer Russia outright being conquered and Marshall Planned into something completely different, but I would accept the other scenario if the outcome could be decent. In this case, Ukraine gets back Crimea and all other territories, plus security guarantees.