Iran cannot defend itself. They begged Trump to stop the bombing. They are weak and vulnerable and in no position to re-neg on the deal. In just one day they would have complete chaos if US forces were to re-engage.
There will be a deal trial period and if it doesn't work, Iran has no plan B. They don't even have any speed boats left. They cannot fight back. Their sanctions won't be lifted, their financial restrictions will be reinstated and their oil will be blocked, while bombs are raining from the sky. They have no real allies in the region. They have no real options.
If they stick to their jihadist guns and revoke the agreement, then there is no piece of paper that will cure this. However, if they stick to the deal and give up their uranium and nuclear weapon efforts, then this is the best way forward for preservation of life and property. They could join other nations in peace and get wealthy, or return to spreading jihadist terror and suffer the consequences.