Even without the reinforced doors, how would the passengers or stewardesses even know that there was a problem until seconds before impact? Under those conditions, passengers would have no time to do anything. Once you get a suicide agent as a pilot, the only way to stop it is with air defense. But even an F-16 is useless against a suicide jet that picks a target along it's general flight path -- not enough time to react.
The only real solution is to fire all Muslim pilots and to ground all Islamic national airlines.
That being said, the pilots should be armed (if they choose to be).
What about Lufthansa or Air France with Islamist French or German citizzens as pilots? How about a "shoe bomer" type. Jamaican-English with a Islamic bent?