Religion

They have religion to bind them and nationalist on top of that.

Creating a sense of persecution and paranoia has never hurt a group, esp. when that's extended to include past wrongs and present distress, coupled with preaching to the group how virtuous and superior they are. The harder you push against them, the more virtuous and wronged they appear to themselves, and the more they stick together.

The only risk Iran really faces is the fact that they're a reduced empire, with a number of conquered people in one border. Azeri, Balochis, Arabs, Kurds, and numerous smaller ethnicities. The Persians have tried to create an "Iranian" ethnicity out of that but still have to put down the occasional wave of Balochi and Kurdish separatist violence, or even Arab separatism in Khuzestan. If the state's hit hard enough, some of those groups may push for more autonomy or independence, but even then they're still mostly Shi'ite and "Shi'ite nationalism" has been a big deal. Instead of three glues--ethnicism, religiosity, and xenophobia they only have two glues. If the minorities teamed up, maybe they could do something, but they're not going to.

(While the Arabs and Azeri could be handled by melding them with adjacent Azeri and Arab countries, the Balochis and Kurds are a problem and would require carving up existing states. The Balochis would gobble up SE Iran and SW Pakistan, and the Kurds we've heard about a lot. Most of the other ethnicities as countries would resemble Lesotho, little enclaves that have scant viability but still have nominal independence and get some respect simply to avoid offending one or another group. Otherwise it would get and deserve the same amount of respect as Andorra.