All of the unruly passengers, bomb attempt(s), and assorted goofballs that even started to act up in the last two years have been generously pounded into submission by unarmed passengers before they had much of a chance to do anything. In fact, the terrorists couldn't even get 4/4 on the same morning as soon as people figured out what was up.
There is always the odd chance that a hundred or two people will meekly sit by and knowingly let four or five with 1" blades give them a face full of concrete and glass, but don't bet on it.
And I don't want to bet ~3,000 lives that the 'odd chance' will happen again. Yes, I agree that there have been a few cases where ONE disturbed individual attempted to take over a plane, and he was taken down. But, we have not seen an attempt by multiple hijackers (in a coordinated effort) either succeed or fail since 9-11.