There's no reason to go higher. 4k at most sizes in most living rooms is already at a point of diminishing returns. 8k TVs do exist, but nobody bothers making content for them. There's no appetite for it.
I agree 4k is enough for TVs, but not for computer displays. 27"@4k still looks pretty bad on a desktop display. For desktop display, 220 ppi (27"@5k or 32"@6k) is perfect.