Without the deal, there is no reason for them not to make nuclear weaponry. As it stands, they have stuck to the deal and not tried to overstep themselves, the deal has even helped their country a lot. If the deal goes under, they will create nuclear weapons right after.
The Iranians had a sweet deal, but they issue is their own internal competing factions. Most of the country is educated and no fan of the Republican Guard to Mullahs nor their power play in Saudi , Syria, Lebanon and Yemen. Trump is also getting pressure from Israel and the Saudis to hit Iran hard in the pocket.
The EU won’t be able to stick with it’s promise to continue the deal even if the US backs out. Here’s why: The reinstatement of US sanctions will limit any European company from US markets if said companies continue free trade with Iran. The business perspective of intentionally opting out of such a huge market simply won’t hold up.