You are ignoring the argument. You are in fact building the strawman for an excuse to point cameras at everyone for the duration of the flight.
This isn't about detection and isn't about triggering a sleep mode at all. It's not even about over-engineering for this particular function, because this particular function is not the point. What it's actually about is doing everything they can conceivably think of to monetize engagement tracking, while continuously pointing a camera at your face as you watch movies and adds.
It doesn't make my post a mistake for not accounting for every last thought you had about a different conversation. And if you need it to be clear, by over-engineered I do consider these unnecessary and unwanted.