Hacker News

To counter this threat, they instituted two positive reforms: bulletproof, locking cockpit doors, and armed air marshals. No longer would pilots respond to a threat in the cabin of the aircraft, allowing the attackers to control the plane directly or indirectly, and an air marshal on board can eliminate any actual threat to passengers.

The actual reform came a little earlier. The actual reform was "Let's Roll".

Prior to Flight 93, the assumption was that the visible hijackers had a hidden accomplice with a bomb. Hollywood told us that anyone trying to be a "hero" would get everyone killed. That cultural mindset was entirely responsible for 9/11.

"Let's Roll" was the attitude change on Flight 93, when they realized there was something worse than getting all the passengers killed. "Let's Roll" is the idea that we are each personally responsible for providing our own security and protecting those around us. "Let's Roll" is the recognition that appeasement is bullshit.

Air marshals are just more security theater. They aren't likely to be present when needed, and they certainly can't be trusted to "eliminate any actual threat to passengers".