I think you might be counting on way too many decision makers to remain rational and wise enough to avoid and defuse non-trivial situations that want to escalate.
In the 8 billion times 8 billion potential nuclear exchanges that universal nuke would enable (64 quadrillion!), there is at least one scary bad one. At any time of day. That you are standing too near.
Please acknowledge my wisdom here. (Hard unblinking stare. Wiggling finger wanders toward red button… “Kind person, I have no interest in harming you, but my sources inform me you have at most one nuke. So I must be prepared to be first. Please keep those hands where I can see them while I only, and with full peaceful intent, rest my finger here.”)
if everyone had a way to completely disintegrate the planet no one would be above another