I may be incorrect, but I've heard varying reports that film holds a resolution of almost 8K. On bluray scanning above 2K probably wouldn't make much of a difference
From what I've seen, movies that get 4K or 8K scans end up looking far superior to their 2K counterparts on Blu-ray. As everything is downgraded to 1080p, it seems like that shouldn't be the case, but it is.
Sony is currently releasing gimmicky "Mastered in 4K" Blu-rays that supposedly look better on 4K TVs. Yeah, whatever... BUT, they look much much better than the previous releases, which were 2K masters. Check out Ghostbusters, Spider-Man, etc.
Gladiator was remastered in 4K to replace the terrible first release, and the result was amazing.
Plus, a problem with 2K is that it's not quite 1080p. 2K is slightly bigger than HDTV horizontally. One option is to scale the image down slightly, but that introduces artifacts and jaggies. That rarely seems to happen. What a lot of studios are doing is cropping the 2K image down to 1080p so that the pixel ratio is still 1:1, but you end up with noticeable information missing on all four sides.
When a movie is in 4,6,8K, the downscaling is much cleaner.