It is extremely unlikely that humanity will be destroyed in a nuclear war. It is completely unclear if the nuclear winter is going to happen in the first place. It is also completely clear that billions will die.
However, there is no other way. Remove the illusion of MAD, and you are essentially making dictator for life the maddest person who has nuclear power. If we don't retaliate, it only means that Putin or his successor or whoever is willing to throw the first nuke, is going to rule. No dictator can be overthrown, cause you know, nukes.
That's why MAD should not be an illusion. feck, I think it should be automatic.
And saying that, I am overwhelmingly sure that I would die during the first week of it. But there is no other way.