That is the rub, they just aren't going to quit and the IRGC doesn't actually care about anyone but themselves so they'll let the entire country burn to the ground as long as they stay in power.
Both of their armies (Regular and IRGC) are trash and would be slaughtered during ground combat operations against the US. Even the Iranians understand this which is why they would sacrifice their ground force in order to buy time for their Basij (the IRGC militia to rise) and the Quds (their actual Special Operations guys) to go to ground. So taking the country takes about a month, and then trying to get rid of the Basij/Quds would take years. The real hardest part is finding someone legit to take it afterwards that the people would support. The Shah's son thinks its him, I'm not so sure.
The have a decentralized structure that is designed to to withstand gaps (i.e. deaths). The cost to that is that the people at the top don't know what the people at the bottom are doing because they don't always have direct communication. That is why the top Iranian elected leadership is often surprised when the IRGC shoots at tankers, no one tells them what they are doing.
Actual authoritarian regimes are serious in how they deal with people. People in the West like to call people like Trump an authoritarian but they have no clue what they are actually talking about. In serious places you are executed and your family put in jail when you step out of line. And then your Auntie is rewarded for turning you in.
I would agree, there were better ways to take care of this. I have a theory that Trump did not go into this willingly and that Bibi forced his hand on this adventure.