The metric of "has Iran detonated a bomb yet" is far to simplistic. What we should care about isn't how quickly Iran gets a bomb, but the probability that it ever does.

To see this via a argument to absurdity suppose that we had an alternate deal that was magically enforced and said "Iran will cease development of nuclear weapons for 10 years, and then we will give them an ICBM or two with warheads on top".

And, I mean, it's certainly possible that it was just a bunch of yahoo's and the deal was effective. It's also possible that the USes various spy agencies have information that made people legitimately concerned about the status quo. It would be outright negligent of the government to tell us (and thus Iran) which it is.

But they were on pace to have a bomb by now. And they don't.

So the deal is, at the absolute least, better than not having a deal. Or at least it was until we unilaterally tore it up after getting what we wanted.