So it seems the "good as Retina" point is somewhere between 122 dpi and 163 dpi.
"dpi" (or, more strictly
pixels per inch, PPI, but that's only an issue when comparing with print) is widely cited as "resolution" in computing and photography, but when you're talking about the resolving power of optical instruments (as in, say, the Mk 1 human eyeball) the appropriate measure is
angular resolution - i.e. the separation of the "dots" divided by the viewing distance. You can only define "retina" just in terms of PPI for devices like phones, tablets and maybe laptops where you can reasonably predict the viewing distance.
If you want to take St Jobs at his word - the "retina" display on the iPhone was 'over 300 PPI
viewed from about 12" away' - if your external display is 24" away (which is about where my iMac is), then it becomes "retina" at 150 PPI.
The math isn't rocket science, but there's a handy calculator at
https://www.designcompaniesranked.com/resources/is-this-retina/ - which gives the MateView as "becomes retina at or over 21 inches".
Of course, retina = ">300 ppi @ 12 inches" is a debatable definition pulled out of the air by Apple, but for print media, 300 dpi (= ppi for monochrome text/line art) at "reading distance" has been the "standard" for some years since.... oh, wait, since Apple pulled it out of the air when they made the Laserwriter
5K will never happen because it's uneconomic to fabricate panels at non-TV standard resolutions.
Well it did happen - with the launch of 5k displays from Dell, HP, Philips etc. some years ago - and then it unhappened when those all sank without trace. Probably complicated at the time because they mostly required a kludgey dual-stream DisplayPort connection. If there aren't enough "early adopters" to start the ball rolling, then a technology won't get off the ground.
Thing is, 5k is just a bit better than 4k but (IMHO) not enough to be worth extra hassle and expense. It has
two selling points: (1) it's a "sweet spot" for MacOS, since it is a 2x scaling from the "2.5k" that has been used in iMacs and cinema displays for years and (2) you can edit 4k video at 1:1 and still have space for control palettes etc. around the edges.
(1) is irrelevant for Windows PCs where "full HD" 1920x1080 has been about the most common format for years - and, anyway, Windows is more flexible about UI scaling than MacOS.
(2) is pointless unless you have golden eyeballs that can appreciate 4k moving video in a window at < 27" diagonal (and in the recent past you were probably using low-quality proxy video anyway) - better to have a large-screen 4k TV hung on the wall just for video preview.
I've got a cheap 4k 28" sitting next to my 5k iMac and - yes - the iMac is better resolution, but it's not
that much better (once you allow for the fact that 4k screens with far better colour than mine are available)... and "looks like 2560x1440" scaled mode is pretty good unless your workflow involves climbing on the desk with a jewellers loupe and doing A/B comparisons. Pre-M1, there may have been an issue with the Intel Mac Mini having the GPU grunt to support scaled displays, but that shouldn't be an issue now - less if a M1 Pro Mini appears.
27” 5K panels will continue to be fabricated as long as the 27” iMac exists though.
Maybe not for long - because unless something goes very wrong at Apple, the 27" iMac is due to be replaced during 2022. Looking at the 24" M1 iMac and M1 Pro/Max MBPs, Apple are quite happy to go with custom sizes/resolutions so the '27" 5k' bit (which is already pretty much an Apple exclusive) is likely to change. I doubt we'll see a massively larger iMac, but I'm sure they'll up the ante a bit. It's quite possible that the last 5k panel to be made is already sitting in a container ship somewhere.
