It isn't really necessary to have actual terrorists or hijackers these days. All you need is for someone to "disrupt" a flight, and then the authorities will do the rest. Just arrange for flights to be "disrupted" in a group of major cities, and you can literally bring air traffic to a standstill, with no need for bombs or terrorists or hijackers or anything. Each disruption costs millions and sends people into a hysterical, self-destructive frenzy, so your goal is achieved with minimal risk and expenditure on your part.
It's a classic case of the cure being worse than the disease.