If you put a 1080p display next to a 4K display, the vast majority of people won't be able to tell a difference outside HDR or better backlight technology for brightness. Unless you're like, two feet away from a huge screen, functionally its just not gonna matter to most people. Ignoring HDR, if I played the John Wick blu-ray and John Wick 4K UHD on the same display, functionally it'll be about the same, unless you're sitting up close and actively lookin for things, which isn't the way most people enjoy content. Furthermore, if I put on John Wick 4K, John Wick 2 4K, and John Wick Chapter 3 4K on the same display, a lot of people will say the sequels look better. And yet John Wick 1 was finished at 4K and the sequels were finished at 2K. The resolution matters only so much.
Steve Yedlin, cinematographer on all of Rian Johnson's films including The Last Jedi and Knives Out,
did a very exhaustive demo on resolution. He goes from 2K alllll the way up to 65mm IMAX cameras scanned at 11K. He goes through them randomly and even in the uncompressed demo you can download its virtually impossible to say "oh, this is the 2K version" or "ah, this must be the 6K version", which is the point. There are so many other things that go into the visual pipeline of what an image looks like, and functionally, the resolution just isn't that important past 2K.
