Clearly you have not seen some of the insanely high pixel densities found in smartphones, some double the 220 PPI of the 5K iMac.
Well, it's not that simple. First, you look at the smartphone screens from a much closer distance, so a higher ppi is more noticeable. However, it doesn't stop there. Not all ppi is the same. For example, pentile displays need much higher ppi to match the sharpness of non-pentile screens. So, a 500ppi pentile AMOLED is not the same as a 500ppi LCD, because they use subpixels. A photo-aligned LCD like the one on an iMac 5K is not like a small 5.5 screen with a supbixel matrix scheme. And I think even LCD screens of that size use tricks to make the whole process less taxing. That is why a phone can run a high resolution screen while a, say, MacBook Pro can struggle with it - because it's not the same. One is a mass market consumer device and the other one is something intended for professionals as well. Also, Macs use better and more demanding supersampling methods than mobile devices.
So, I think we're still far away from 8K screens on iMacs or even Mac Pros. It would be very demanding even for top hardware and the benefits are questionable for anything other than 40+ inch screens.
Apple doesn't do something just to have a "first!" moment. Currently, I don't see a need for desktop screens higher than 200ppi. My Retina Macs look just as well if not better than my iOS Retina devices, unless I get close enough to strain my eyes. For something that would be hard to notice, the tradeoffs of 8K would be too much. Unless they are actually working on a gigantic retina screen (but then again, I don't see what a super-expensive and problematic 40" 8K screen could do that 2 5K 27" monitors couldn't).
I could be wrong, but I don't expect 8K Macs anytime soon.
