I don't see apple rolling out an 8k display when the ink is not even dry on the 5k (figuratively speaking). We are just received the second generation 5k iMac and there's not much out there, to fully leverage the 5k display, i.e., how many 5k videos are we watching?
8K is a long way off being mainstream, hell they have had little luck with 4K and broadcasters still don't support 4K in all but the smallest fashion. Yes 8K will get here but you've got to be looking at 8-10 years before it's mainstream...