How cockpit rules changed after 9/11 terror attack & how did they help avert a similar tragedy?

Before 9/11 terrorist attacks, cockpit doors were as flimsy as lavatory doors, and while there were rules to prevent intrusion during the flight, they didn't protect from hijackers. After 9/11, FAA mandated hardened cockpit doors that can be unlocked from inside, to resist physical attacks. In the flydubai incident, Indian pilot unlocked the door from inside, averting a 9/11-like incident.