Iran needs nukes to defend itself from a nuclear armed aggressor. Everyone needs nukes for that reason. Greenland needs nukes to protect itself from the US.
Probability of nuclear war rises with number of states having nukes. It's best to keep that number as low as possible, so I would not think it wise for Greenland to have nukes. It would not be a sin for Iran to have them, though, given Iran's allies aren't exactly offering a nuclear umbrella.
with extinction technology, i don't know what the answer is. i think you either need a high level of trust and cooperation between all wielding parties which never goes away, or you need a singular world government which has no reason to arm itself with such a thing.
the stalemate situation where all enemies have a gun to point at one another so that nobody fires a shot is crazy. that can't be the solution.