If you accept that MAD only goes one way because one side doesn't care why can't that side conquer and rule the entire world after all for each nation the others nations plight is less than existential so they ought by that reckoning accept each nations plight until it is their turn.
Even when invaded being conquered represents a lesser loss than being blown up so one would logically either preemptively accept defeat or at least withhold your nukes while the enemy grinds you down using traditional means.
It would seem vastly more rational to work together to destroy the aggressor. If you really think current leadership will nuke you rather than be defeated then remove that leadership and see if the next leadership feels the same way.
Will we allow Putin to murder Ukraine, then Finland, then Sweden, then what? How many other lives is one mans life worth?