Having read your recommendations for MAC OS and 4K or 5K displays, so unless I'm misunderstanding something, it seems to me that Windows (8.1 and certainly 10) is easier to get right. Caveat, I've not tried it myself on a 4K or 5K display, so others should correct me if I'm wrong.
On Windows 10 there is a clear separation between fonts/icons and screen resolution. They can be set seperately to optimal values. The Screen resolution should be set to the display resolution (so images will display 1 on 1 per pixel), assuming that the Graphics card supports that. The text/icons/etc. can then be scaled independently on the operating system level. Applications since 2015 have mostly become High DPI aware, and will use the OS level settings, unless they offer additional/alternative settings, so unreadable tiny controls that take an effort to click on, should not be an issue anymore and there is no need to emulate a lower resolution and show blurry icons.
I can imagine. We're getting close to the Human Visual System's acuity limits, especially when we view from slightly more than normal reading distance.
I agree. I do assume that the exact half physical screen resolution (to avoid having interpolation at fractional pixel positions) is needed because the specific MAC display drivers do not support the actual display resolution? Otherwise, why not set to the exact same resolution as the display?
It is not a limitation on display drivers per se. It's a design decision on how to support HDPI screens back when the original MBP was launched with a 2880x1800 display. The MBP before that had a screen resolution of exactly half that in each dimension, 1440x900. The MacOS at the time was enhanced with scaling for the new screen such that fonts, text, menues, etc. was scaled (but rendered in the full screen resolution) to look the same size on the new retina screens as on the older MBP's lower resolution screen. New API's were made for retina aware applications as non-retina aware appliscations were simply scaled up and therefore did not look as sharp as it could (1 pixel from the application was mapped onto 4 pixels on the screen). There was also introduced a scaled option so you could choose a different size for the fonts, menues etc. (aka. "looks like" resolution) that could be set to e.g. 1680x1050. You could not choose any resolution you would like and only the ones presented in the scaling options. The "looks like" 1440x900 was the default which it has been on MBP's until the new 2016 model where the default strangely enough was changed to 1680x1050. There have been complaints about this since there is a tiny fuzzines introduce3d since the virtual screen would be 2x1680x1050= 3360x2100 which would be scaled down to 2880x1800.
Now with 4K and 5K monitors this approach has been continued so for a 4K monitor you can choose a "looks like" at 1920x1080 which will map into a virtual screen resolution of 2xfull hd= 4K (3840x2160). This maps then perfectly to the 4K screen resolution and is completely sharply rendered. But on a large monitor like a 32" monitor the fonts and menues, etc. look quite large although the images pixels will be rendered exact. So if you choose 2560x1440 to scale the fonts to the more correct size then the virtual screen will be 5120x2880 which will be mapped on the 4K screen so therefore looks a little unsharp. It's not a lot, but if you zoom into 1:1 in Lightroom to judge sharpness you will see a slight unsharpness. If thats ok 4K screens are fine on Mac's. But notice that the virtual screen resolution in this case is exactly 5K! SO using a 5K monitor the mapping is perfect. That is the reason that Apple chose the 5K resolution for the 27" iMac (I should that I believe so since I have never seen a statement from Apple that this is in fact the case). When using a 4K screen you can also choose "looks like" at 4K in which case everything is mapped to 4K exactly, but then the fonts are pretty small. When using a 4K monitor there is also another option: For normal e-mail and web browsing use a "looks like" scaling at 2560x1440 which looks good for that and keep the monitor preference open at all times. Then switch to Photoshop or Lightroom (or any other image processing app) and switch to the monitor preferences and choose 1920x1080 and then switch back to the image processing app. The the image pixels will map perfect 1:1 to the 4K screen and visual elements like text will a bit larger but maybe ok. Since the MacOS scan change resolution while all apps are running this is a workable solution although one I didn't like too much.
So you could call this a limitation, but really it is a design choice that Apple made. We can argue about this choice and if this is the best choice or not, but that does not change anything
Although not very detailed here is a description of the Windows scaling options https://www.tekrevue.com/tip/windows-10-display-scaling/
and the scaling of the fonts and menues etc. It says that you have to log out when changing the size which is different from the Mac where this is not needed. See above about this. I assume this logout is needed to render all fonts in the new chosen size so they don't look fuzzy.
So don't get me wrong. 4K on the Mac does look good, but if you really want to have perfect rendering of images in 1:1 you need to know how it works.