[contingencies]

Airplane systems require physical access.

... to potentially only one of the components within the system, at any point in its lifetime, across the entire supply chain and all build, test/verify, operations and maintenance processes.

(Edit in reply to child: Yes, obviously "the components within the system" means those actually connected, not a number 3 sprocket in seat 63E's incline mechanism. You have re-iterated my point.)

[constantcrying]

False. Most components are not connected to any of the relevant busses.

And if you had control over the specific component you need the plane is already compromised, whether the bus is open to spoofing or not is an irrelevant question.