One, Iran is huge. Iran has a population of 90 million people, which is a lot more than Israel and Ukraine. this means Iran has basically huge recruitment potential, huge conscription possible.
In a long war, sure. It takes months to produce a quality soldier; while Iran technically does have peacetime conscription, anecdotes I've come across online suggest said recruits aren't given serious military training. It's mainly there to make military-aged men a captive audience to regime propaganda for a while.
But why, you might ask? Well, in large part for the same reason they'll be reluctant to call upon the draft now: they don't want a potentially hostile fifth column having access to quality training and weapons. People who've chosen to make a career out of the army/IRGC thus far have been almost entirely regime loyalists or people with no moral scruples who just want to earn a paycheck, but a true, blind draft would fundamentally alter the balance of power inside the country.
Again, however, even if this were not the case, the regime would have to survive the next few months to mobilize said multi-million man strong army.
two, Iran has ability to fight back, with many drones and countless missiles in storage.
Their missile capabilities will fall off within another day or two assuming this isn't already the case, because the US has been targeting their launchers (missiles are useless without some means of firing them), barring a minority of launchers that they're hiding underground.
Drones are a more serious problem, sure. The "shooting down a $200 drone with a $2 million rocket" problem hasn't been solved yet. But I don't believe this one asset is enough to upend the power imbalance between the US and Iran; for one thing, their purportedly huge drone stockpiles haven't translated to an overwhelmingly high volume of drone attacks happening all at once. All a large stockpile in their case means that they can keep the current pace up until their ultimate defeat.
three, Iran will fight back, because at this point it became obvious its "do or die" situation.
As I've discussed at length, this might be true for the army but not the citizenry at large, who overwhelmingly either hate or are ambivalent toward the regime. Having bombs dropped on their cities for a couple of weeks is a scary prospect, but living the rest of their lives under the boot of thugs who barge into hospitals to summarily execute wounded men and women is even scarier. Maybe a small minority of bootlickers actively prefer this but they're not "Iran" at large.
Four, Israel will be primary target. the most obvious target Iran could attack is Israel, and by far much easier target than US bases because US bases can be missed easily, Israel is difficult to miss.
As an American, that's no skin off my back.
Iran's armed forces are among the largest in the Middle East, with
approximately 610,000 active-duty personnel and 350,000 reserves, totaling nearly 1 million personnel.
Same was true of Iraq in 1991 (they actually had a bigger and more modern army than China's) and that didn't stop them from getting steamrolled. Granted, there's no guarantee this'll prove the case again, but what I've seen so far suggests the IRGC is indeed a paper tiger. They're on the verge of losing control of the skies, a sizable chunk of their navy is at the bottom of the ocean, their supposed cyber threat hasn't amounted to anything yet, etc.
The only strong cards they have to play are blowing up stuff with drones (which has had the consequence of making their previously neutral Arab neighbors join the fight against them) and closing the Strait of Hormuz to oil tankers. And the latter only really matters in the event of a long war.