When they commandeered those 767's they basically took control of the biggest, baddest heretofore untested semi-organic weapon of war to stride/soar over the earth. This was all very symbolic to the enemy. They see the airplane as the modern white steed; it is their nomadic mode of transport and warmaking. They are going whole hog into avianics. Simple, copied but a highly lethal and destructive means to pinpoint an attack. They will do another synchronized airline assault just to show us that we are helpless to stop them. I wouldn't be surprised if they didn't perfect a way to subdue a cabin of people with chemical delivered via aerosol and therefore be able to overwhelm the cockpit. We have got to do more to thwart their ability to do something like this.
If you did something and it was hugely successful, wouldn't you do it again?