BGutzman, passengers stopping hijackers is a new dynamic. Prior to 9/11, the paradigm was that hijackers wanted to go someplace, make a statement, and/or demand a ransom. Common wisdom was to just ride it out, and let it be dealt with on the ground.
September 11 changed that.
Flight station doors will no longer be opened. Why is this a big deal? On Flight 93, the passengers fought back but lost - as the fight was going on in the flight station. Jam a body or two into a control yoke or the rudder pedals, (or even a body part, or for that matter a pencil) and you'd be amazed at how an airplane will come out of the sky.
If a knife is held to the throat of a flight attendant, these days, that door will still not open. The bad guys gaining access to the flight station is expected to kill everybody, now - so that access simply will not be given.
Without access to the flight station, terrorists need either cutting tools (in the sense of cutting through reinforced doors, not box cutters) and the time to employ them while the passengers and flight attendants are trying to stop them; explosives (blow the flight station door, or just blow up the plane), but those aren't easy to sneak aboard - and passengers are looking for it; or firearms (could handguns be used to get through the door - I doubt it, but suppose things are possible; they could definitely be used against critical aircraft structures).
Allowing people to bring firearms into the cabin with a certificate for proof of training actually would enable the bad guys. Unlike in your normal city or countryside, gun control can actually be made to work quite well in a vehicle that requires security checks for all who enter.
Note: at the time of 9/11, the box cutters the terrorists used were legal to have on board.
So, since 9/11, name one occasion where passengers have tried to stop a problem aboard the plane with bad results, BGutzman.