I agree that it could use more resolution, and have been saying so since the iPad 3 was released.
At that size it really needs a 3072×2304 display. (a 3× retina display rather than 2×)
This would be 396 pixels per inch and should have more than enough resolution for most, if not all people.
Despite the iMac 5K having only 218 PPI, the iMac screen looks far better than the iPad Air 2.
You have to factor in both viewing distance and display resolution.
Most people have a 1080p television now and if it's over 44" in size, it has less than 50 PPI. But people typically sit 10ft away from the screen, so it doesn't look pixellated. For many people there would be absolutely no benefit moving from 1080p ("2K") to a 4K display, as the screen is not large enough, or they don't sit close enough to benefit from it.
That doesn't mean it applies to
everyone though. If you have a particularly large display, or sit closer than most people do, you will notice the difference in resolution. (I certainly do)
All of the retina Macs should have enough resolution for the distance that they are typically used at.
I
would like a resolution increase on the MacBooks, but that's more functional than for image quality reasons.
With the MacBook Pros the workspace was reduced from 1440×900 to 1280×800@2× on the 13″ model, and 1680×1050 to 1440×900@2× on the 15″. It's ridiculous that the MacBook Air has more usable workspace than the Pros.
I'm very glad that they did not make this mistake with the iMacs, and waited until a 2560×1440@2× display was available, rather than settle with a 4K display. (1920×1080@2×)
The screen on my iPad Air 2 is gorgeous. If I really enlarge the text I can see "questionable" pixelation, same with my iPhone 6. At normal font size and distance it is perfect and my vision is fine with glasses. All of my Apple devices are retina as I can't stand pixelation.
I am not targeting you specifically here, but it surprises me that so many people get this wrong.
When you make text larger on the screen, each character now has far more pixels to draw the letterform. This makes resolution differences far
less noticeable.
Where you really see the difference with higher resolution displays is with
small text, and non-western letterforms in particular.