Anti-missle technology on commercial jets = RJ
This type of thing ranks right up there with SDI in its ability to be effective.
First off, even a direct strike by a stinger or similar missle is not capable of bringing down a jetliner, even with a direct hit. The hit would have to be so precise to bring it down or even disrupt the aircraft the odds are in the extreme.
Second, the technology of an automated anti missle system doesnt exist and even if it did, would probably bankrupt a lot of airlines if they were to retrofit their fleets with it. Airlines could barely afford to retrofit their cockpit doors after 9/11 and all reluctant to retrofit their center gas tanks to prevent explosions, something else on the table in this arena.
Third, even if current technology such as flares and chaff was used, it would be ineffective. This is becuase this technology was designed to be used in combat situations, when pilots were focused on fighting the enemy and averting threats such as missles. It is not designed to be used when pilots are focused on the business of takeoff and landing.