There is no going "back" to absolutist monarchism because it was actually a highly evolved (over centuries) form of government that eventually became the political norms of Europe today. There are a lot of ways to go forward to an isolated dictator driven by a terrible fear of his own citizens however.
I meant an absolutist monarchy. This is really a dying form, though not yet totally dead.
Paradoxically, republics of today seem more vulnerable to authoritarian turns than the monarchies which survived the 20th century.
Probably because an elected leader has more legitimacy and thus can demand more political power, sometimes too much power. In a constitutional monarchy, there is always a psychological split between the sovereign and the prime minister or whoever gets elected to executive power, and one-man-shows are less likely to succeed.
If Iran turns into a monarchy, it will be a very formal constitutional one, like Spain.