Sorry maybe a misunderstanding, I was a projectionist in the distant past (including 35mm). With an arc lamp at the right color temperature and the highest quality lenses I don't see what we were missing that was there on the film. BTW I don't like what they are doing to some of the old films on channels like MGM either. Personal preference in there too, I won't waste any more space here.
Briefly: By the time the film is printed for distribution it's some generations away (e.g. negative->interpositive->internegative->release print) away from the original.
A way in which the quality is reduced is that the "resolution" (or rather "modulation transfer function") is degraded with each generation, added to which is "gate weave" etc., plus the projector's lens (which, even if excellent, the total system performance is the loss in each stage multiplied together.)
Some published work has found that, as (properly) projected, release prints are below "HD" resolution, albeit this has been the subject of some debate (e.g. over the quality of the sample "release prints" used.) Either way, it is clear that it's not "4K," and it's certainly much less than the original camera negatives.
The problem, then, is that there is an (unreliable!) memory of what a movie actually looked like in cinemas/theatres, as well as the fact that release prints were never accurate copies in the first place, and that available reference material (i.e. stored interpositives or release prints) may have substantially degraded over the years.
Also, in the digital domain so much more is possible (e.g. in colour grading) that there is always the question of what the movie would have looked like, had the same tools been available at the time it was made.
It's a minefield, so I'll leave it there--and good film transfers to digital distribution are another story.
I'm sorry that the above wasn't altogether "brief" enough; however, suffice it to say that the
parallels with audio are, I trust, more than obvious. (e.g. Generational loss, "resolution" of vinyl vs. 16-bit PCM, available reference recordings that provide accurate impressions of the originally intended sound, poor quality source recordings/transfers/mastering, etc.)
(N.B. I have the greatest respect for "old school" film projectionists, who worked hard to provide the very best possible quality of presentation and maintained high professional standards. A few individuals who uphold that tradition still exist today.)