Aw, cmon, there would no nuclear World War. What we're talking about, here, is a limited exchange. Less than we detonated in the atmosphere on our own soil in the 60s. Iran would spend one and Israel would spend about 10-15 and that'd be that. I'd be far more concerned about the world wide economy in the toilet for a decade after all ME oil had to be extracted in radiation suits that I'd ever be about 'humanity perishing'.
But you're correct. They won't be allowed to attain one. And no, the Mullahs would not survive the response. Plate of glass comes to mind. Including the mountains.
While the US may see it as a "limited" attack, you have to consider how other countries would see it, especially Iran's allies who would stand up for its defense. That is where the situation becomes unpredictable. What do you think the risk is? Zero? One in ten? One in a hundered? Or Certainty? What risk of all-out nuclear war should we be willing to take as long as we have an alternative?