First up let's clarify, I'm not trying to side one way or another. Apple is a huge business, and the Apple TV is multi-faceted now. I'm merely debating why it's not as black and white as "iPhones support 4K recording (not even enabled by default) so the Apple TV should support 4K". As I mentioned in my first reply to you if all the Apple TV did was stream media, maybe they could've - but it isn't, and they haven't.
Also, me as a tech guy - I'd have been happy with part 4K (for media) and part 1080p for apps that can't support smooth 4k. But this is Apple, they market their devices in a specific way. I just can't see them offering 4K, not having their iTunes content available in that res.
Remember the Apple TV is part gaming machine, I could easily spin this and say you're downplaying that aspect too much - we have no idea what the long term plans are.
4K TV ownership is obviously going to grow. But let's look at the available data, Netflix had this to say last year (
http://blog.netflix.com/2014/05/netflix-now-streaming-in-ultra-hd-4k.html):
"Market researchers predict that consumers will buy a million Ultra HD 4K TVs this year and even more in subsequent years. We expect it will likely take up to 5 years before Ultra HD 4K becomes mainstream; when most of the TVs on store shelves are Ultra HD 4K."
So in about 2019 the majority of people (in major markets?) will have a 4K set.
And you're right not adding a feature because some can't use it make's no sense. However your wording masks the truth. In this case it's a feature that only a small percentage can take advantage of. And that makes little business sense.
Internet speeds are on the whole way too slow for 4K streaming to be viable for most. Again a Netflix source, as they are in a great position to know this market:
http://ispspeedindex.netflix.com/ - current averages in major Netflix markets is ~4Mbps. That would need to quadruple to hit minimum 4K requirements, and improve 6 fold to get close to recommended 4K speeds.
The iPhone profits are irrelevant, if you follow Apple they don't seem to play the subsidisation game across hardware. The way they structure their financial reports too, it's as if they treat each line of products as a business in itself. They could all stand independently.
There are so many high level pieces at play. If I pretend to be ever so reliable market analyst, my wild guess would be a 2017 or 18 launch of the next Apple TV with 4K support. That makes sense in terms of user base (TV ownership, better internet speeds), available content (more than just Netflix, and Amazon if they want to be on Apple TV - most likely iTunes catalogue would have to be on board) and technical (SoC to power it, 4K specs mature). But that's just my unqualified guess.