Iran kept developing its ballistic missiles and drone program even after the deal was signed, and a decade later, Iran has hundreds of thousands of drones and 20,000+ ballistic missiles. A thousand ballistic missiles do as much damage, if not more, than a single nuke.
It also leads to the interceptor problem, namely, it is not possible to stop thousands of missiles coming towards you, and eventually you run out of interceptors and get overwhelmed.
It was a really dumb deal, and this issue was called out at the time, but nothing was done about it. It's like an agreement between Mom and two kids that are fighting. Mom tells one kid, "Okay, promise not to kick your brother!" and he agrees. So he starts learning to punch instead.
