Like for instance in plenty of older SciFi films there is a strong influence of the Cold War; are you saying that now that the Cold War has passed the relevance of films produced during that period is diminished? You could make a case for that, I guess, but then how would that process be any different for books or music than it is for film, as has been suggested in this thread?
I don't disagree that fewer and fewer people will watch films made before 1970 as time goes on, but I think that has more to do with a lack of familiarity with the current audience. Frankly I also think that the access to these films is nowhere near as easy as it should be, the studios should be opening their vaults online to get people to watch older films - I bet they could make a tidy profit with that.
It's different from books and music, because in books and music the reader or listener is invited to imagine the settings visually as they see fit. Problems of errata, cultural confusion, settings, don't exist so long as the person consuming the art is creative enough to fill in their own blanks. With film, the entirety of text, sound, and picture are there for you and you can interpret them but you cannot bend them to your own imagination in any way. They are inflexible in that way, so as certain things (like the Cold War) become less culturally relevant, as they no doubt will be as time passes, fewer and fewer people will be able to connect with the narrative in a meaningful way.
Your other point is probably more to the case and a better general argument, is that lack of access to older film is a more serious constraint on the audience.
I think both of these things present solid reason why, for example on the IMDB list that is entirely composed of modern people's opinions, the list is overwhelming biased towards films made in the last 20 years. Because both the cultural relevance of many films made prior to the last 20 years is lost on the majority of the film watching audience and because even if these people want to see older films they are constrained by availability.
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Pour the wine, hold the grind, quarter to nine, let's go.