But everyone would have to have those, or else the first country to develop them would have an insurmountable advantage. A country that has both nuclear weapons and the ability to block all attacks including nuclear would rule the world, or at least dominate it without opposition.

Or, everyone would have to have a strong sense that nuclear launches weren't possible.

I'm thinking along the lines of grey-hat anarchists constantly attacking everyone's nuclear capabilities. "If nobody is super, everybody is super."

Hence the local maximum.