We pretty much all agree it would be impossible for the terrorists to again hijack a plane in the same manner as before. The passengers will fight back.
So their next option would be to maintain fake ID, pilot or stewardess uniforms, etc. and board as crew. Then make their way into the cockpit, at which time the door is secured behind them.
Securing the cockpit, as the airlines have done, may actually now give them an advantage, because if they can get into the cockpit, they can keep everybody else out and fly the plane into "whatever."
Part of why it's better to have a door that can be broken into with enough effort, but an armed pilot. Even if the terrorists get into the cockpit and lock the door behind them, if there are more passengers than the pilots had bullets the terrorists could still be stopped.