The only mindset I refuse to have is to pretend that these films still exist in a very tiny sample size as they once did. I don't know if Citizen Kane was once one of the best movies ever made, but I do know I once thought Mystery Men was one of the best movies ever made and seeing a lot more movies corrected that, but for some reason Citizen Kane will always be one of the best films even though over 1000 times more movies exist now than did back then (worth noting in particular than a lot of the "best" films that were made during WWII suck, just like how music sucked during the Korean war). And if you do give olds films a chance, you might find that some were better than Citizen Kane anyway. I'm pretty sure you only brought this up because I said Citizen Kane felt dated compared to an even older film, so what's the actual problem here? The real problem is with specific time periods, regions, and studio controls. Hollywood sucked in the late 90s and it sucked from WWII through the McCarthy era. It's got nothing to do with films being old.
I think there is a common mindset that films have somehow plateaued, sometime between the late 1930s to the mid-1980s or so. That everything before X movie, well, it was still people learning, and then afterward, it was people just copying what was finally mastered. In the first few decades of film, people were still learning how to integrate narrative, still learning what the best length and pacing for narrative is, still learning how to integrate sound, camera effects, cinematography, visual effects, what have you. Then there was this period where all of that stuff was "perfected", and now everything else after that is just riding its coattails, so to speak.
The most common defense I hear of why Citizen Kane is the best movie is because it was a pioneer in so many different camera techniques and effects as well as visual effects, and many of those techniques and effects are still used today. And the logic goes, if every movie going forward was highly influenced by that, can they really be said to be "better than the original"? In some ways, they can only exist in reference to it and owe a lot of their qualities to it. And if many great movies have come out then, they still can ever pioneer the same things again, just like the wheel can never be "re-invented" (this argument probably applies to the top 5 or 10 "best films" since obviously Citizen Kane didn't pioneer
everything). Art always exists inter-textually and it's impossible to judge it without reference to how it was influenced by other works. I agree with this argument insofar as Citizen Kane should get credit for what it accomplished and influenced, but I don't think that means it's necessarily better than all the other films it influenced.
I don't think I'm really qualified to say what is or is not the greatest film ever; I'm not even sure a category like that isn't too subjective to exist, which is probably why most people go for the technical and legacy argument when defending Citizen Kane, which are largely objective and can never be changed no matter what other films come out. I like it, among a lot of other old movies, though it isn't one of my favorites. But I think for many people there really just is no way to avoid a sense of age-based bias when it comes to films. It seems to me this is the trend with a lot of art forms, there always seems to be a certain point where it was the best and everything before it was building up to that and everything after was just copying it. Whether that's Baroque music or Victorian literature or whatever (painting gets a little different, I grant). At least film has only been around about 100 years or so, who knows where it will really go when it's been around for a thousand and what we'll consider to be "old" at that point.