...Data caps are a real thing for the majority of NA homes and a fact of daily life across Europe.
All you can eat internet is over. With more homes abandoning TV package deals ISPs are making up for the loss by forcing tiered internet service. 1TB of data is enough for most people when it comes to internet usage and video steaming at 1080p. But 4k streams would eat through data caps in a matter of days. Does anyone recall the lawsuits that surfaced over WiFi Assist? An iTunes store that supports 4k (plus the apple TV to consume it) could have unexpected repercussions.
While I support you with your other arguments, I am not sure what you are speaking of, since we are talking about people that mainly watch from their homes and data caps for homes are mostly wired connections with unlimited data, like it has always been. Also, since you would still have a choice between 720p, 1080p and 4K (2160p), I do not see any repercussions at all.
4K streams are always
optional, all other resolutions will still be available anyway. You kinda make it sound like suddenly all content will be 4K, which is just not correct.
Abandoning the idea of 4K just has no arguments other than not willing to do it, since Apple currently does not see the benefit for themselves.
The A8 is more than capable of delivering 1080p, since on the 6 Plus the chip needs to be downclocked (for battery life reasons, which is not the case for the Apple TV) and also has to render at a way higher resolution (2208x1242), which gets downscaled afterwards (which also is not the case for the Apple TV).
The only thing it seems to miss is either API support or hardware support for H.265/HEVC (or similiar) and HDMI 2.0/HDCP copy protection and a physical connection for full HDMI support.
My personal opinion is that Apple will certainly introduce 4K when they see their benefit, but before, it probably won't happen, which is a shame since Apple has all the talent, the experience, the ecosystem and availability of new devices every year that could make this great.
But we will have to live with that and search for alternatives (Google Chromecast Ultra, Android TV 4K), even if we don't want to, since Apple seems not to be interested.