The terrorists would have been hit with so many frangible rounds that they would have been pulverized. Pun intended.
Actually if the passengers were armed, we would have shown that terrorism is a losing proposition against Americans.
Which is why I don't like the disarming of Americans who fly.
I would accept some type of licence requirement for a person to be allowed on the plane armed: say documented training in firearm use, no felony convictions, no crimes of violence (not sure if there are no non-felonious ones).