This is true for a negative, but for prints, especially release prints, there is rarely much information above 4K, and generally many films struggle to represent 2K resolution.

The blu-ray often has more detail than a release print, but not always and not consistently.

It is a bit hard to explain, but being able to scan from the negative lets you capture detail that never made it to the print, but sometimes there is fine detail in the print that is lost when you downscale to 1080P.

Grain is definitely a ‘fault’, it is the very definition of noise that obstructs the signal, but in many cases it was ‘used’ to help hide matte lines, blend separate elements or breathe a bit of life into a static shot. In other words, it was a known limitation, and was factored in to the final look of the film, and that makes it worth preserving in many cases.