If we come to a nuclear agreement with Iran, I guess we can look forward to reviving all the nuanced discussions of whether Iran's nuclear weapons status is imminent and whether we have sufficient proof to say Iran is cheating.
Iran will go nuclear and we will do nothing.
Oh, and then we can revive talk of nuclear deterrence. Because we all remember how we considered that such an ideal situation during the Cold War.
Our options aren't good after all this time. Yes, we can be grateful that Iran hasn't gotten nukes as past worst-case estimates have given us (See? isn't the imminence debate fun?)
Striking Iran is now more difficult because Iran's knowledge base and institutional base is so much larger that Iran will have less trouble rebuilding what we wreck.
Add in the impossibility of knowing how much of Iran's nuclear program is actually in other countries (as it is with North Korea and was in Syria before Israel blew up a nuclear reactor being built by North Korea), and we have to admit that striking Iran to set back Iran's nuclear ambitions is more difficult now.
Of course, I've always held that regime change is the best way to remove Iran's nuclear threat; and that attacking Iran just buys time.
But I'd rather have that purchased time without a nuclear Iran, if that is our only choice.