Based on back of the envelope math I did based on known ideal viewing distances, most TVs are too far away to be able to get all the benefit out of even 4K. Something between 4K and 1080P would be the “ideal” balance between bandwidth and resolution.

For monitors, the maximum resolution is a bit behind 4K (since monitors are closer than TVs), and I suspect that’s why Apple offers a 5K iMac.

I didn’t estimate this for mobile devices.