> Not to mention the changed calculus for the passengers if a hijacking were to occur.

If there were no air marshals, and the cockpit door was replaced with a 1970s-style beaded curtain hippie door, I would still expect future hijacking attempts to fail.

Before 2001, a hijacking was almost guaranteed to be an inconvenience for the rest of the passengers. A diversion to another airport, being stuck on the plane while negotiations happen, etc. After 2001 every passenger is going to assume they're being commandeered as a missile. There will be tons of resistance from 1A all the way to 33F.

(Strong cockpit doors are still a good idea. I'm not arguing otherwise. But the changed calculus you mention is enormous. It alone I think would make successful hijackings a rare occurrence.)