The shoulder fired systems are the largest threat. They are easy to transport and conceal. They can be smuggled across borders. Terrorists can hide near an airport, in fields, forests or waterways, and surprise an airliner landing or taking off.
Imagine a dozen of these, using Al Qaeda tactics, so the shoot-downs are timed to happen simultaneously across the country.....a dozen airliners shot down at the same time....3000 dead, our air system shut down, no defense possible, our economy shattered.
Which is why 9/11 could well be described as “amateur hour”.
It is folly to believe they did not learn from the exercise nor develop new tricks.
Next time it will likely be just before the elections.