mrklaw,
Retina-level pixel density is not impressive any more.
The rest of the industry cought on, it's just a name. And a "retina" display brand is not just resolution, it's the quality of the display. And Apple keeps pushing here constantly.
Even if one can't actually see pixels, a lower PPI display is still inferior (all other things equal) because higher PPI yields a sharpness that is easily perceptible
Not really. I can't see the difference between an iPhone 6 and 6 Plus without putting the screen really close. Most people can't either. There is no easily perceptible sharpness at normal viewing distances, other than the fact that 4K sounds cool on a specs sheet. I'm not saying you can't notice it, just that it's not easily perceptible by most people.
Regarding developers, while I am not one, I do believe that today's development tools provide for adaptability, which was previously not possible. This is, I believe, how universal apps (iPhone and iPad) can exist (i.e., apps can shape AND size themselves.).
You're right about the part of not being a developer - but you're wrong about the rest

You're talking about adaptive layout which alows apps to stretch across the larger and smaller screens while retaining the relative resolution of assets in points. However, increasing the pixel density would cause all rasterised graphics to change real-world size (in cm/inches). What you need is upscaling, which would require higher resolution assets, which requires new graphics. Not to mention that in order to allow that same graphics to downscale on older devices properly, Apple would need to do a 4X resolution increase (a 9.7" iPad would need a very large resolution of 4096x3072 which is larger in vertical resolution than even a 5K iMac and 2x larger than your typical 4K screen).
TLDR: iOS9 adaptive layout wouldn't help with the resolution increase, the resolution would have to be really large and the performance/battery hit huge. One day, perhaps, when the trade-offs are minimal. Don't expect that day to come anytime soon though.
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Why stop at 4K? Wouldn't 8K be even better? 4K is so last year!
No, it's not a matter of opinion, it's a matter of 4x slower performance and lower battery life vs something you see when you squint your eyes.