I made that argument for North Korea, but not for Iran. A nuclear Iran would be very bad, though we might have to get used to it anyhow, because ultimately the only way to truly prevent a nation from acquiring nukes is military force, and IMO that is an untenable option for us with regard to Iran.
As to your specific point: Iran was never going to let us inside to inspect. Like most autocracies they are far too paranoid (and too proud.) so it was either take what we could get or continue the sanctions, eventually without our allies. We took what we could get on the notion that opening up trade with Iran is a greater opportunity to eventual peace than isolating them is. I think that’s a good approach in general in the nuclear age, for China, for Vietnam, for North Korea, for Cuba, for Iran. It would have been a good approach, IMO, for Saddam’s Iraq.
As an added note: this insistence on inspections of nuclear facilities has never really worked during the last several decades that we have tried it. Most nation-states are too prideful of the indignity involved- we certainly would never allow international inspectors to come here and tell us what to do.