[owlbite]

I'd guess the aim is to be secure on all components (most of these things have their own processor(s)). If you can compromise one component you can move from there to compromise another one, until you get to something worthwhile.

I don't think my main concern would be three letter agencies (they're going to find a way in to your average consumer one way or another). Probably more likely some organized crime gang backdooring cheap replacement screens and using that to perform an attack on financial data or similar. Attacker doesn't have physical access to the device, just manipulated the supply chain.