The bit that worked though, both for this and Richard Reid's shoe bomb, was that passengers - not air marshals - noticed the terrorist trying to set light to something in the main cabin area.

All it takes is a terrorist bright enough to try and set light to their bomb in the toilet rather than sitting surrounded by other passengers while fiddling with a fuse and lighter, and the system has failed. These attacks are failing because the chosen attackers are stupid, not because the system is effective.

Air Marshals. 4.2 arrests per year average, $200m per arrest. A very, very expensive comfort blanket.