Most videos are probably going to stay aimed near 30-40 degrees of width. So while we already can do 8k or higher pretty easily, 4k is enough for a huge amount of content. Even 1080p can hit "retina" at 30 degrees, if barely. And while that threshold for "retina" is flawed it's pretty good for video content.

We'll see how much 360 content shows up, but the size gap is so big it basically needs to be handled entirely differently. A 16x9 screen, 40 degrees wide, is only 2% of a sphere.