If Iran's stale regime comes apart, and it will at some point, the entire country may follow.

Few Americans are aware that Iran is in fact a multiethnic empire.  Native "proper" Farsi–speakers, located mostly in the middle of the country, account for only half the population, identical to the ratio of Russians within the USSR at the time of its dissolution.  Other Iranian peoples (Kurds, Lurs, Gilaks, Baluchis, and others); Turkic speakers (Azeris, Turkmen); and Arabs live on the nation's periphery and account for Iran's other half.  All of these groups have their own home provinces, or home regions across several provinces.  Azeris alone are as much as one fifth of Iran's total population, such that there may be far more Azeris in Iran than in neighboring Azerbaijan.

While there has been some persification — Ayatollah Khamenei's father is said to have been of Azeri descent — most of the periphery is likely no more (and perhaps less) "Iranian" than the smaller republics of the USSR were "Russian" in 1991.  Moreover, irredentist or independence sentiment is already strong in the Arab, Kurdish, and Baluch areas, the latter two of which are not entirely safe for Iranian security forces.  And we can surmise that students protesting in Tehran would find little in common with the rough bunch who would rise to power in some of the provinces should central power collapse.

Instead, the country could become another Libya.  With the dissolution of the Islamic Republic, where before stood one monolithic regime, Uncle Sam would have to develop relations (good or bad) with perhaps a dozen quasi-states led by tribal warlords or other interesting characters, as well as deal with new ISIS affiliates in Sunni areas of the country.  Iran's depressed yet relatively modern industrial economy — though it is perhaps not as inter-regionally integrated as that of the USSR — would fragment and shut down, much as did Soviet industry's geographically severed bits and pieces in 1992.

Given Iran's location and its 80-plus million people, we simply have not seen a geopolitical shock on this scale since the Evil Empire died.  If it happens, America can forget about ever leaving that part of the world.  For this reason, President Trump is wise to seek to cut the ayatollahs down to size without pursuing "regime change."  However, his economic strangulation of Iran, well justified as it is, may have unforeseen consequences beyond the simple war scenario that is on everyone's mind.  The risk is that the country passes the point of no return before the too proud ayatollahs and their goon squad resolve to save their skins by capitulating to Mr. Trump.

In 1991, we saw a powerful yet economically and ideologically bankrupt super-state run out of steam and disintegrate.  Yet the USSR was more or less cleanly divided in a largely amicable fashion along clear, pre-existing borders among its 15 nominally (and by 1991, in some cases genuinely) autonomous, self-governing republics.

If Iran implodes, and it probably will within the next few years, things won't be nearly as clean.  At that point, no one will remember the hipsters with their slogans on the streets of Tehran.