I suspect you are correct. As an Air Traffic Controller of over 20 years, my perception of what could have been done with the WTC aircraft is grim. In the time it takes to humanly recognize the nature of an incident like this, it is over. The idea to shoot down any aircraft that has a communication failure, loses its transponder or behaves erratically for a few minutes would mean the downing of a dozen aircraft every day.
If NORAD was set up to monitor all the daily flights for erratic behavior, it would have a staff nearly as large as the FAA, and the results would be the same or worse. Worse being the complications that arise with overlapping duties or missions.
The true cure for this, sadly, is removal of the privilege (or right) for private citizens to fly aircraft. Id est, fewer planes means closer monitoring and restrictions on routes and proceedures. This could be coupled with mandatory public motorized mass transportation, etc., etc. In short, how far do we restrict our freedoms to be "absolutely" safe?
This I disagree with.
First, a fighter that goes up to intercept one of your dozen aircraft doesn't have to shoot it down. All of us (pilots) have been taught to recognize the international sign for interception and that if such interception should occur we should follow the intercepting aircraft to a field to land.
Second, the 9/11 planes weren't exactly operating as a confused 172 pilot might if he were IFR and had an electrical failure. The WTC planes were at altitude on a western course 100 miles north of NYC being piloted by very experienced pilots. Such pilots who were really in trouble at that point probably would have headed for Rome AFB. I'm sure that all sorts of alarms went off five minutes after they broke off communicantions and turned south. My understanding is that planes were scrambled, but that they just couldn't get to NYC (from Otis AFB on Cape Cod) in time to do any good.
ML/NJ