I used to think the same, but displays beyond Apple's Retina do look better. Have a read of this excellent article on the topic:
http://www.cultofmac.com/173702/why-retina-isnt-enough-feature/
Based on that, you'd need an 8k display on the rMBP for true Retina.
Ya I've run across that article before. The idea is that some people can see better, and therefore we need to fix the limits beyond where they are currently claimed to be. But how common is this? Screens for computers are mass produced and so unfortunately if the average person can't see any better than Retina, it's going to be a tough sell to go beyond since for most of those buyers it will be a waste of money. So what percentage of the population sees better than 20/20 (with correction since that's how we view the screens)?
Also, the studies that show human visual acuity can reach as good as 0.6 arc minutes are in optimal lighting conditions with patients who have no visual aberrations (or worse, when the experiment directs the imagine directly on to the retina of the eye in order to determine the limits of visual acuity, bypassing the natural route of the image). Those are not real world settings. That might be well and good if all your consumers are under 20, but as we age our visual acuity tends to degrade.
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But, this definition of retina is something apple came up with, "300ppi at 10-12 inches is retina", i believe if you do the math from an
arc minute it is much higher
You've got things backwards here. Apple saw the studies saying that the limit was 1 arc minute and did the math from that basis. For 1 arc minute it yielded a result of 300ppi at 10-12 inches. So they built a screen to go beyond that and called it Retina (it came in at 326ppi).
furthermore these calculations are all based on 20/20 vision being normal, BUT
studies have shown that normal vision is closer to 20/16 or 20/12, that would increase the distances listed above
True, they are based on the 20/20 metric. However, which studies have shown that normal vision is closer to 20/16 or 20/12? Can you provide them? I've read that wikipedia article, and tried to follow the links to investigate, and all those links either don't work or send you to pages that do not support the conclusion drawn, i.e. that normal human vision is better than 20/20. So maybe you can provide concrete data, if you happen to know of it?
Vernier acuity is interesting. I can't comment on that yet since I haven't look into it. But if everything checks out, you may have pointed to evidence which suggest that Retina and 4k class screens are not the desirable end point.