What consitutes "high PPI" depends entirely on viewing distance (and apparently a made up thresold). Do you really think people use the 21.5" iMac 4k (219 ppi) at the same distance as the 32" 6k (218 ppi)? Unlikely, I'd suspect.
If the former is "retina" at a closer distance, it can lose PPI and maintain the same angular resolution to your eyes at a greater distance. But then you lose integer-multiple scaling which macOS does not like very much.
I think they want to make 8k the next display standard (it will be for video at least, eventually) but the cost is massive and the benefit is slim, so 5k or 6k will be the high end for now. The percieved pixel density increases linearly while the number of pixels (to produce and feed with data) increases exponentially.
…Plainly speaking, viewing distance enables millions of humans to get away with less than ideal resolution for TVs that panels for stuff like monitors and phones cannot.
The ergonomic optimal distances for monitors and phones need higher resolutions than TVs to enable ideal sharpness that again has standardized criteria to be considered high pixel density towards 4K not being enough for a monitor with a panel
larger than 24 inches.
OSes/software intended for mobile devices and desktop computers behave in the matter I described to output high PPI content or not.
This is indisputable and has been the case for well over a decade.
Again: A panel needs a DPR of 2 and above and a PPI of around ~218 at minimum to be considered a pixel-dense screen.
8K is what TV manufacturers are pushing as the concerted effort for them to sell newer TVs.
It’s common knowledge by UXers, engineers, and Human-Computer-Interaction (HCI) computer science experts that resolution is overrated in representing sharpness of a screen.
Rhetoric text such as “4K” is easier and cheaper to consistently market than pixels-per-inch (PPI), device pixel ratio (DPR), pixels per degree (PPD)—you know the things software actually use and care about the most to decide whether or not a screen can be considered a pixel-dense-screen.
5K for 27” and 6K for 32” is the minimum for large panels to catch up to where mobile devices have been for over a decade.
4K resolution beyond 24 inches is ill-equipped to provide high pixel density—it’s the McDonalds of resolution targets to shoot for on a large display.
It’s definitely convenient and prevalent—doesn’t mean it’s good for you!
8K, 16K, and higher is obviously more ideal—as well as necessary for large displays to catch up to the pixel density prevalent on mobile devices today—but you’re right the costs can be prohibitive for manufacturers and some end users to justify