What does "retina" mean exactly? From what I can tell it's a general Apple marketing term rather than something concrete. Are 4K monitors unsupported by macOS? I've been using one with my MacBook for a while and I haven't noticed blurriness.
From reading the comments in these threads and some additional resources, it does seem that the problem is not whether or not the display is “retina” per say, but that macOS is designed to run on high density displays and can have problems with low density ones. My mistake I guess.
I think the point that Apple should have better support for a wider range of displays still stands. It is not unreasonable to want your general purpose, professional-grade computer which costs thousands to work correctly with a variety of displays and resolutions, especially when lesser machines do it better.
At this point I’m just whinging bitterly though, the blurry icons are not the end of the world and buying new displays or a new computer just to fix them would be silly.
So on phones it has usually been about 300dpi. On laptops and desktops it's been something over 220dpi.