There were many forces involved in the Revolution against the Shah besides Khomeini’s - including MEK as I mentioned. I haven’t made any argument as to the legitimacy or illegitimacy of the Iranian regime. Since you brought it up, there was no guarantee which faction would come out on top, especially as it became clearer that the Shah’s days were numbered. MEK, for example, enjoyed plenty of “
popular support” amongst leftist student movements and the professional class.
There’s a compelling case to be made that Khomeini wouldn’t have been successful in the first place without
US help. The would-be Supreme Leader essentially played US leaders like a fiddle, painting a moderate, introverted and US-friendly picture that was very different from the “global Islamic Revolution” he would launch once taking power and American hostages; to say nothing of the purges that
killed thousands of perceived political threats as Khomeini consolidated power, and the
universal suppression of any media or institutions deemed incompatible with theocratic rule.
MEK was just one of the various political factions on the receiving end of these purges. The terroristic and guerrilla tactics they would adopt in response is what you correctly call terrorism whilst somehow dismissing Tehran’s actions and history as “a propensity for irregular warfare.” The difference between MEK and the Ayatollah is that the former lost.