You're close to a good point raised by another person in the replies, that another hijacking attack is unlikely to occur with all the new safeguards, but you're supporting your argument with all the incorrect reasons.
The primary culprit that allowed such events to occur was a policy of cooperating with hijackers. That policy had been tested and proven effective dozens upon dozens of times before to counter act plane hijackings. The problem wasn't that the airlines were "trying to save a buck" by not reinforcing their doors or that "the patrons weren't all packing some sort of weapon" it was the very simply policy of cooperation. I mean heck a couple guys with box cutters stand no match against a mob of people - whether they're packing knives or not.
Now the correct people (air marshals, pilots, flight crew, etc) are trained to deal with situations like these. That's why it won't happen again.