If Boeing decides to cut costs by using a single network for their entertainment, communications and flight control, who is there gonna be to tell them no? You would assume there is some oversight by independent security researchers who review these planes security, but this assumption seems unsubstantiated.

In reality, how is this separation of critical networks looking like exactly? MAC address filters? Are they air-gapped? I would venture to guess, nobody that isn't bound by an NDA knows.