Printing is becoming less and less common, IMHO, as most photos are now viewed on video screens of one kind or another.... Tom Hogan suggests in a chart at the bottom of this page:
http://www.dslrbodies.com/cameras/camera-articles/image-quality/how-big-can-i-print.html
... He suggests that m4/3 sort of tops out on "excellent" prints at about 13x19. ...
I really think the big camera makers need to focus on other parameters of photo goodness, rather than resolution -- stuff basically out of reach of iPhones and compacts. Better low-light response, better color response, a wider range of affordable specialty lenses ... I went to a Ryan Bingham concert Sunday night and took some iPhone photos, in which you can sort of tell what is going on, although you really can't make out facial features. A 12-year-old D3 would have blown it away. That's where the strengths of enthusiast/pro cameras lie, I think -- the hard stuff.
That all makes a lot of sense to me. I would love to see real data, but my guess is that a great majority of (us) users of mainstream format ILC's (up to APS-C) are doing little or no printing, and what printing they (we!) do is almost always no bigger than 13" x 19" or A3 (which is 11.7" x 16.5"). I fit that profile anyway, so I'll say "we" from now on.
If so, probably the most detailed display we see is what we see on modern 4K or 5K monitors. For example the 21.5" 4K and 27" 5K iMac displays are both about 218ppi, about matching the 200PPI of dye sub prints that were reckoned to match traditional photographic prints for detail. So for detail, those displays about match traditional photographic prints of dimensions 19'" x 11" for the 21.5" 4K model and 24" x 13" for the 27" 5K model — bigger than we are likely to print, and with far greater dynamic than any traditional print.
I am not saying that the pixel count of those screens is enough for such viewing: Bayer CFA pixels arguably have less detail than the full three color pixels of a screen can display, so it could make sense to use about 300PPI camera data on those 218PPI displays. Let's say 288PPI, to fit Thom Hogan's magic number for "excellent" quality! That would come to about 20MP to fill the height (but not the width) of that 16:9 5K screen.
And some of us like to zoom in on parts of a scene occasionally, so there's some value to even more pixels.
BTW, I continue to be skeptical that our eyes can make use of 8K, at least under video viewing conditions of significantly greater than one picture height from the screen, even in the best seats in a cinema. It might be that there is some visible improvement from 4K to 5K and maybe even from 5K to 6K, and the way the video technology and standards go, the route is to just double at each step, so that "more than 4K video" is got by jumping straight to 8K — and then massively compressing for transmission, and even for initial recording.
There will be no 16K video!