A popular classic like Cassablanca have endured due to strength of story and acting, something which ages much better than "groundbreaking at the time".
By the time most viewers today see Casablanca they'll have already seen most of the scenes ripped-offed, parodied, homaged, or referenced. For the first time viewer it won't feel like the original and instead will feel like an old black and white copy with a strange style of overacting.
The same happened to me watching The Wizard of Oz. Suddenly I remembered all those scenes from the Simpsons which I did not get before and was like, "now I get it".
There is an episode of Star Trek DS9 where a character almost literally quotes the speech at the end of The Third Man (given, incidentally, by Orson Welles). Had I not seen The Third Man, I would have thought it was just a stupid monologue, but knowing where it came from changed the tone and gravity of that scene completely.
http://en.wikipedia.org/wiki/Pierre_Menard,_Author_of_the_Qu... - where a book now being rewritten identically word for word is, of course, a completely different book ...
Few under-30 year olds have even given The Beatles or Pink Floyd a proper listen either or know of Ferris Bueller's Day Off - I'm serious, mass cultural memory is not very long.
But I think you'll find that all of the above will be mentioned in discussions of 20th century movies and music long into the future. Citizen Kane definitely stands the test of time.
But then, Borges' reasoning would apply to most anything not made recently. There's a very small cadre of writers/creators aware of anything not produced in their lifetime.
(Furthermore, Borges could not guess at the time that Citizen Kane would be anointed as one of the show pieces of "the seventh art" for a few decades - a fate that could probably have been bestowed upon a few other contemporary movies, but was not.)