Gaming aside, 4K is desirable even on <30" displays, and honestly I wouldn't mind a little bit more pixel density there to get it to true "retina" resolution. 6K might be a sweet spot?

Which would then imply that you don't need a display as big as 83" to see the benefits from 8K. Still, we're talking about very large panels here, of the kind that wouldn't even fit many computer desks, so yeah...