Games are also different than films/tv visual wise in my opinion, with games you either have AA or don't for the jaggies.. i don't think that's an issue with films. Like with games you might get things that suffer more from obvious aliasing like railings etc, with film it is just all the same pretty much. So games (probably?) benefit more from higher resolutions than pre rendered films would, maybe that's why I've seen people say 1080p Blu rays look good on a 4K telly? Also like others have said the sitting distance matters, with TV at normal sitting distance i guess you could go quite large with a 4K display, i think i read somewhere that 65" was the recommended largest size. Hell even my 65" 1080p TV still looks quite sharp to me and i usually sit around 8+ feet from it.
I just saw a video on the newer Dell 8K monitor as well where the guy games on it, basically saying it's pointless over 4K visually. But again size/distance comes into play. Overall it looks like 4K will probably be standard for a long time, like Richard (Digital Foundry) recently said, you would need a huge screen to benefit from 8K. In fact i don't even see how they will get people to buy 8K TV's if 4K is already at the point where more isn't needed? I genuinely wonder.
Personally though I'm happy at 1440p 165hz currently and I've only had it a little while, as a couple of others have said the responsiveness is much better in any game really with high hz, on the controls and on the eyes. I'll go 4K 144hz when the performance is there in a single GPU and prices aren't sky high.
4K 60hz is doable but you can't hit a constant solid 60 fps in demanding games.