First. it has escalated very quickly—almost immediately—from a protest over prices to a demand for the regime to fall. This is unusual in such events. It suggests a strong underlying opposition to the regime, only needing a trigger.
It has spread across the country very quickly. We'll see if this continues. Day one: one city. Day two: seven cities. Day three: dozens of cities all over the country, including the capital. Day four: ? It begins to look like a genuine popular uprising.
Reports are that security forces are reluctant to fire on protesters, and the regime has responded immediately to the protests with concessions. The law requiring women to wear the hijab in public has already been rescinded. This is a sign of weakness. The regime does not seem confident they can rely on the security fores to do their bidding. Revolutions always win when and because the security forces refuse to fire on protesters.
Iran seems to be in the classic pre-revolutionary state, as analysed by Crane Brinton. There is a huge cohort of educated young people, and high unemployment. The Guardian quotes anonymous "Ali" as saying, "Every year thousands of students graduate, but there are no jobs for them." Exactly the situation right before the French Revolution. Recent economic hopes have been dashed, leading to a sense of frustration. Iranians had reason to hope that, with the nuclear deal and the lifting of international sanctions, things would get better economically. They have not; thanks no doubt in large part to the fallen price of oil. It is this sort of frustrated hope, and not a long-term general decline or political repression, that inspires people to turn out in the streets and demand change.
Unlike in 2009, this time, the US government is likely to back the protesters. And the protesters have been told so, in the Trump administration's immediate announcement. Regardless of any material aid the US sends, this is an important psychological factor. I have also heard it said that both Israel and Britain have important intelligence capabilities on the ground in Iran; if they put them at the US's disposal now, effective aid might indeed be possible. Israel has every reason to do so. Britain should probably want to as well: they need a trade deal with Trump now that they are pulling out of the EU. Besides, it matters to their prestige to demonstrate such capabilities.
So, in sum, I think odds are decent to good that this actually ends the regime.
What happens then?
First, who takes over? Most likely, the military. There have been calls in the street for a return of the monarchy. But in any event it is not going to be another Islamist regime. The Islamists and the Muslim religious authorities will have been discredited by association with the fallen regime. The next regime is almost certainly going to be secularist.
If the Iranian regime falls, and a secular government replaces it, this becomes another huge win for Trump. Right after his signal win over ISIS.
Iran has been propping up Assad in Syria. With Russia now withdrawing, if Iran pulls back, Assad probably falls.
In the meantime, the pressure comes off Saudi Arabia in Yemen. Hezbollah loses its funding in Lebanon. It should be a great boost for Saudi Arabia, who need one. A lot of pressure comes off Israel. A lot of pressure comes off the government in Iraq, where Iran has been funding factionalism. The effect on Saudi Arabia's restive Eastern Province and Bahrain is hard to foresee. A new government may be inclined to be less interventionist, but a more attractive new government may be more of an ideological magnet for Shia populations there.
With ISIS down, Saudi Arabia having declared its intent to turn in a more secular direction, and with fewer funds to promote Wahhabism abroad in any case, and the Islamist regime out in Iran, Islamism may soon be on its last legs. The Iranian failure will tend to discredit Islamist movements elsewhere in the region. It begins to look like a failed ideology; like Baathism and Nasserism before it.
Presumably the new government would scrap Iran's nuclear pretensions. The program dictated from the streets apparently will be money for the Iranian people, not for foreign adventurism. “Iran First.” This takes pressure off the US in turn, and allows them to focus more completely on North Korea's nuclear threat.
Of course, we may see some other dominoes topple, if Iran's government topples. Just as in the “Arab Spring” a few years ago. Then, similar demonstrations soon appeared not just across the Arab world, but also in Russia and China. The results of the Arab Spring have been disappointing enough to work against the same thing happening again in the Arab world. But maybe again in Russia or China.