4K is actually what the early RED cameras recorded in. It'll be weird if we ever move past that, would we just upscale those movies? I mean, we couldn't "remaster" them like we do with old movies and Blu-ray.
Honestly, 8K (I think that's what TGWTDT and TSN are) already looks amazing to me, but it may just be the shock effect of not having grain. If everything were digital, and then 16K came out, I wonder if I'd notice.
1080p TVs actually have more detail than people can make out based on how far back from it they usually sit.