But that a nuclear Iran would be a terrible blow - not least to Iranians forced to live under a theocracy newly empowered by the Doomsday weapon - is indisputable. [emphasis mine]
Middle weight powers that can assemble minimally credible retalitory detterants are dominated by superpowers' nuclear arsenals where the Singlurly Assured Destruction strike is fundamentally a rounding error in total throw weight of the superpower. An Iranian nuclear arsenal is only valuable against the US when the US has minimal stakes in a confrontation, and since the Middle East has the swing oil supply for the next generation or two, my assessment is that there are very few scenarios in which the US will have minimal perceived interests at stake.
Nuclear weapons are not useful in holding onto power from internal political pressure and opposition. If this was the case, then the fragile and weak leadership of the post-Soviet successor states would not have agreed to have returned their nuclear weapons for disarmament to Russia, and the political/military/economic pressured white South African regime would not have had to have bowed to the inevitable demographic fact that non-white people are the overwhelming majority of the South African population.
There is a decent probability that if the United States does not allow the Iranian conservatives to successfully cast the US as a credible foreign threat over its bellicose rhetoric on the Iranian nuclear program, the rally around the flag affect that has historically occurred within any nation-state whose population perceives a foreign threat will dissipate. In that scenario, the conservative theorcrats' hold on power weakens instead of strengthens with the addition of nuclear weapons.