I think a lot of young people these days don’t necessarily know enough about film (as in, physical film, not ‘movies’ or ‘cinema’) to realise just how much detail 35mm captures. Growing up in the last 20 years myself, I remember TV going from SD to 720p to 1080p to 4K, and digital/phone cameras doing similar. I think it’s feasible that a person of a similar age who doesn’t take a particular interest in A/V (or fanediting, or film preservation, or anything else along those lines) could just assume that the picture quality of movies would have also followed a parallel progression, and that modern Bluray releases of old movies must therefore be some kind of upscale. It doesn’t help that many have heavy DNR giving them a waxy, unreal look.