Re: Come again?
"You can presumably sniff things like EMI, or otherwise detect hand movements. Lots of possibilities here, with interesting precedent in what's been done against PIN pads."
You'd still need context, though. Harder to get without access to the innards.
"Plus your phone has other secrets to protect than just its' contents. Like everything being said in the same room as the phone, even if it's off if it's bugged."
Still need a way to EXfiltrate those conversations, and if the radio chips are also protected, then you'll need a total package. Might as well use a specialized bug in that instance.
"Regarding PIN pads, the VISA EPP standard is not meant to withstand a day or so of unsupervised access, which is what handing your phone in for repair certainly does in a lot of cases."
ATMs have to sit by their lonesome for days at a time. Who within a location actually pays attention to the PIN pads during normal operation? As for techs, that usually points to inside jobs, meaning they have access to key chips. Rogue techs could use side channels like hidden cameras, but again that's close to insider status to get them clandestinely in the machines and outside this context.
"The scenario for DVD/BluRay/etc is to protect the actual digital data, to prevent an exact (high-definition high-quality) copy, not keep the contents per se seciret."
The reason being they have a perennial problem: the enemy only has to be lucky ONCE. Then sharing instantly nulls their economic advantage, and the human condition means people WILL cheat. That's why they've been working on this VERY hard for the last 20-30 years, coming up now with this chain of trust system for the 4K systems (as well as the consoles, which double as 4K players) based on what the phone makers have been doing (and some phone STILL haven't been rooted or custom-ROM'd at this point; ask xda). Similarly for pwning a device. ONE slip and it's Game Over. They have to hold that off for as long as they can.
"If you hand something in for service and don't trust the service techs, consider it pwnd. This is almost a basic law of computing."
But not COMPLETELY. Otherwise, we'd see a formal proof by now a la Turing's Halting Problem disproof, as there ARE real scenarios where DTA must be assumed, so there IS a practical angle.