How long do you think they can keep the UNSC as it is? What are they going to do with a country like India with over a billion people? Things change. Systems that cannot adapt to new changes will be destroyed.
A nuclear threshold state is in no way similar to a state holding megaton nukes. A nuclear threshold state at best can count on a very limited number of small nukes under 50 kilotons that is of no strategic value.
Nuclear armed states have insane enrichment power. Russia and the US for example each have over 10 million SWU of enrichment power. And they usually measure the SWU in U Kg/year. France too. Iran's nuclear program is just a fart in the storm compared to what nuclear armed states do. And Iran will never be allowed to reprocess plutonium under today's conditions.
Brazil, Turkey, Nigeria, Indonesia, etc. will all continue to rise in next decades. Look at where Turkey was 15 years ago and where they are now. Brazil was pretty much the image of poverty 20 years ago and look how much they have progressed. Nigeria has 200 million people with oil reserves and a population that is rapidly becoming educated. Sure, they're a joke now but who knows what can happen 20 years later? Is Iran the same as it was 20 years ago?
North Korea lacks the resources that Iran has. If North Korea controlled the world's energy routes and resources like Iran does, the US itself would beg to mend ties with them. Let alone Russia and China.
Nuclear armed states have developed extremely powerful weapons that are more than enough to wipe off entire population centers. Obviously, they have reached the end and they do not want to go further because there's no point. That's why they are focusing on other things now. I mean look at the Soviet Union. When they tested the first Tsar bomb which yielded 50 megatons, they discarded their plans to test the 100 megaton nukes. Not because they couldn't build it, but because they were afraid of testing it. A 50 megaton nuke would completely wipe off Tehran and leave it only as a name in history books. That's why nuclear armed states are less focused on nukes these days.
It turned out to be a really big issue back at the time. I don't know if it was the Russian propaganda machine or real, but Russia even passed a law about it after numerous children were abused in the US only because they were Russian.
en.wikipedia.org
Believe me or not, many Americans, particularly the older generation, hate Russians to the core. Even more than they hate us.