Well, there's something which could 'theoretically' happen...and then there's a credible threat.
Flying a light a/c delibrately into an airliner COULD happen, but so could an ground-to-air missle attack, or another 'shoe bomb' incident.
The issue is deciding what is CREDIBLE and what isn't, and planning accordingly.
We cannot anticipate and plan for every single possibility because there will always be vunerabilities at some point. There is always unfortunately a 'softer' target.
(JetStreamRider beat me to it!)