The real questions are-
1. Which puts Iran further from a nuke- having 7 years of oil wealth, infrastructure development, missile and weapons R&D and being more of a global power, or being kept struggling under sanctions, suffering from domestic turmoil, actively sabotaged by a campaign of hacking/assassination/etc, and with the threat of intervention if they get too close? On one hand they get closer to breakout during a 7 year delay and hopefully have less reason to want it when that ends and live up to their promise, but without the deal they have farther to go to breakout but reason to try for it immediately.
2. Is it really in our geopolitical interests to oppose Iran getting an eventual nuke? Would the gulf actually be safer if it could reach mutually assured destruction with wealthy, stable and peaceful states, or does sectarian strife and the drying up of oil reserves and the very nature of islam make nuclear armageddon inevitable? We are more ideologically aligned with Iran than the Saudis, and having them as a competitive power allows us to play both sides of the field to keep them against each other instead of against us, as the sunni-shiite war of the past decade has demonstrated.
3. How much could we ever leverage a relationship with Iran geopolitically when it is such a threat to the Saudis and drives a wedge between us and Israel? When the Saudis were overextended and overpumping and could bare no more teeth to Obama he had this opening, but it was only a temporary one from which they've returned, and Israel isn't our puppet state and wouldn't be kept on the leash indefinitely if we went down Obama's path of fractured relations. Whatever benefits we had from the Iran Deal, there were geopolitical costs.
4. Could any major diplomatic deal have ever survived when crafted in such a unilateral way in opposition to majority of domestic legislators in a partisan environment? This was true both for the US and Iran, where opposition outnumbered support. If any house built upon such unstable footing was doomed to fail, a controlled demolition might have been in order.
The benefit of the Iran Deal was temporary peace and stability and the potential that Iran could develop and open up enough to pose less of a threat by the point it reached nukes. The risk was that we were given them free reign to develop their nuclear tech to a point we wouldn't be able to avert a gulf nuclear crisis by force or sabotage come the sunset date when they could have a ~12 month nuclear breakout after 2025 with rapid centrifuge deployment, with ten years and trillions of dollars to put into it. It was our hope that after that point, they wouldn't have the reasons to take the plunge and go nuclear, but they'd wield that threat as leverage and sectarian strife in the middle east made it almost an inevitablity.
The pre-Iran Deal calculation and what we're returning to was the pragmatic calculation that we could make the road to a nuke so slow, treacherous and painful that it would be in Iran's rational self-interest not to go nuclear. Much like how Iraq didn't even try it after the Gulf War. With a campaign of spycraft and assassinations and hacking we could slow their program to a crawl, and threaten direct intervention if they tried to get close anyway. It put the Iranians in the situation of wanting to negotiate reprieve from the sanctions by abandoning their nuclear ambitions- precisely what the Iran Nuclear Deal was supposed to accomplish. But the calculation is one of delayed and inevitable nuclear crisis versus volatile and indefinite conventional crisis. The Iran Deal opponents at the time, Hillary's camp of establishment democrats so prominently among them, argued that we could have extracted more permanent or indefinite solutions instead of the 10 year reprieve setting a much stronger Iran loose after the sunset dates with nothing but their promise and our hope it wasn't in their interests.
I think too much of the Iran Deal debate like everything else is subsumed by petty partisan discord. Its completely mind-fucking-boggling to me that just because Donald Trump is president and did it, I haven't heard a single liberal political commentator on the radio or TV arguing against the Iran Deal even though so many of them did just 3 years ago, even though it was probably just as likely Hillary Clinton would have eventually done the same and I'd be hearing those same arguments in reverse with NPR criticizing the Iran Deal. I think its just not possible to figure out whats really in our interests when the only thing I'll ever hear is if it benefits Drunuld Drumpf or not.