As long-term readers know, I couldn't care less if Iran becomes a nuclear power. There are lots of nuclear powers in the world today, and none of them has used its bomb. Even though it's well over half a century old, mutually assured destruction (MAD) still works. India and Pakistan may rattle their silos from time to time, but they never launch. Even if Israel didn't have enough nuclear weapons to vaporize all their antagonists in the region, Iran still would not risk using a nuke. Hell, even Israel won't use its nuclear weapons unilaterally.
So, what's the benefit of a nuclear treaty with Iran? More than anything, it brings the Iranians closer to the rest of the world. By comparison with their neighbors, Iranians are better educated, less religiously fanatical, and much better disposed towards democracy. After all, it was the democratic election of Hassan Rouhani that made the nuclear agreement possible.
Reintegrating Iran into the world economy will make it more responsive to external influence. Alleviating the hardships suffered by average Iranians under sanctions will lessen the influence of the Revolutionary Guard and other right-wing elements. Further sanctions will not work (see Cuba), and the only other alternative is war, which also will not work (see Iraq.)
The time to normalize relations with Iran is now.