Now that its a problem for them to fly because of scans,pat downs,sniffer dogs or explosive detection devices, you have to ask yourself where can they go that there is a large amount of people and no searches.
They will soon start to think outside the airport or airplane.
Or start planting IEDs along well traveled road way, can you imagine a tanker full of gas under an overpass on a thruway during rush hour in say LA.
Or walk into a mall at Christmas time in several cities all across the country. And blow themselves up.
They will make it as unsafe as they want to.