Harmy said:
Plus, up until about a year ago, even movies shot on 35mm had their digital effects rendered at 2K, which means that not only the fully CGI scenes, but any live action containing even a little bit of digital tinkering, are stuck at 2K, so for example the LOTR trilogy is in the exact same boat as far as 4K, as the Star Wars prequels.

For those little bits of digital tinkering in films shot in 35mm, I imagine it would be possible to recomposite the lower-resolution digital effects onto the original footage? Of course, that involves quite a bit of work, tracking down all the source elements and making sure it looks good. I imagine not all source elements have survived to this day, for some reason we aren’t very good at archiving everything. That’s where LucasFilm seems to do alright with, they have pretty much everything available in their vaults, but they don’t necessarily take the best care of it either.