Apple has long LONG moved out of the 'gamers' market. They have no real intention of worrying about driving anything other than OS and GUI level graphics.
I don't think they (and to be honest, MOST users who don't game) care that their 4/5/8k desktops won't be able to game at native resolutions. As long as it can handle the GUI and your day to day tasks, thats what Apple is for
IMO there is no need for a 5k display, let alone an 8k display. I'd much rather Apple reduce the price of the machine, or put that money to upgrading other tech. within the machines.
If I were going to buy a new rMBP tomorrow and they had a 220 PPI vs a 400+ PPI. I don't care if the 400+ was just $1 more, I would not spend $1 more for a better display. You give up too much performance and are trying to fix something that isn't broken.
For giggles, I'd say a 300 PPI 15-17" display is the stop point for me. But since I'm happy with 220 PPI, I wouldn't even wish for 300 PPI at this point.
It's bad enough I have to worry about Apple spending so much time trying to thin everything out. Now I have to worry about the machine simply powering the huge display. And did you see the specs of the new 12" MBP compared with 2011 versions of the MBP? So we are getting weaker machines anyway, and now take that weakness and go power an 8k display.
If you like looking at beautiful wallpaper, I guess the display will be killer. If you like to do anything else (and I'm not talking about just games), good luck.