The US and friends can't track the thousands of airplanes in the air, much less the 1188 777s, all nearly identical, at the same tine for the months if not years that potential terrorists could hide this airplane until they use it. Reader detects airplanes, it doesn't segregate. You would have to check each airplane, scramble fighters (quicker said than done) attempt communication, receive clearance to fire, find a safe place to shoot the airplane down over, all before it hits something. It is a needle in a haystack, especially if it were done at night.
Boeing 777s can travel 1164 Maurice miles with a full capacity of passengers, London to Hong Kong. They could strike anywhere in Africa, Europe, Asia, the Middle East, or America with range like that.
the attack would be more strategic in the Middle East, Asia, or even Europe.
This is what would happen if MH370 appeared anywhere. They would shoot it down before it could become a threat. I don't think there's a single strategic target in the world that could be reached by a Boeing 777. This is also assuming crazy things, like the landing of the plane at a remote air strip (which likely damaged the landing gear), its refueling and repainting, etc.
This is because we don't have mechanics to fix stiff like this, right?