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Maybe it will have retina! I just can't see how they can implement a 27" retina display and still keep it priced reasonably. I know us Apple users are willing to go a little above for our Macs but a 27" retina display would be MUCH MUCH MORE than "a little".
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Well, first, it's unclear that retina displays cost much more than non-retina displays. After all, you don't pay for LCD panels by the pixel. A retina display will use the same amount of raw materials as non-retina. The only reason why I think it would cost more is because the components of a bigger pixel might be able to "survive" a small manufacturing defect but the same defect might ruin a smaller pixel, so yields of panels might be lower for that reason.
Also, Apple might justifiably call a 4K display "retina" for the 27" model depending on how far away they think people sit from the displays. It would be 163 DPI, which is probably enough to be called "retina" from ~3 feet away, and "only" has about twice as many pixels as the current 27" display, instead of 4 times as many as with the retina iOS devices.