We don't even know that the hijackers brought their "weapons" through security. It is entirely possible that their box cutters were placed onboard through this very channel (caterers).
But, all this crap is just for show, anyhow. Don't think we don't know that. If they were serious about stopping terrorists, they'd let the pilots arm themselves and they'd let CCW holders carry onboard with frangible ammo.
I don't think that this was the case because the letters the hijackers had reminded them to bring their knives.
With increased scrutiny at security checkpoints, assistance from ground crews becomes more likely though.
And then there are all of the businesses that operate between the metal detectors and the boarding gate. Most probably have some cutting implements (restaurants, newsstands, gift shops) for use by the employees if not the customers.
If you are really determined, an aluminum can (from the beverage cart on the plane) could be torn into a sharp edge.