I don't know. State control over people's lives is scary. China is one scary place in my opinion. Imagine they implement the social credit system in Iran and restrict your internet access based on your social credit. It's just scary and will lead to insane brain drain and much more divide between the people and the system.
As for your arguments:
1. I'm pretty sure foreign elements in Iran can easily find loopholes and security vulnerabilities to stay connected with their coordinators. As you said, if there are VPNs that work, those foreign agents surely have them.
2. People in Iran watch Iran International and Manoto via satellite dishes. As far as I know, jamming the satellite signal hasn't been working well against these channels so far. That's probably why they have millions of viewers within Iran.
My guess was that they cut off the internet to prevent a similar scenario like what happened in the 12-day war. I'm talking about cheap FPVs using Iranian phone operators like IranCell. I think this is probably the most important reason for this restriction and it has worked so far. But if this war lasts for months or even more than a year, what are they going to do? Do you know how many people will get unemployed because of this?