What's important to note here is that virtually ever list has 1-2 memorable films, and the rest are basically forgettable junk.
This idea that something like 2001 Space Odyssey was a normal motion picture of its era is very confused
This is why the past is idealised isn't it, because the cream (not always but often) rises to the top and survives and the dreck sinks into oblivion.
But at the same time, certain historical conditions allow for works of art that previous/subsequent conditions preclude.
For my third bloody obvious point, I'd point to TV. If cinema has declined, at least in the mainstream, its surely because TV has become a magnet for the talent that cinema used to attract.
But this is complicated too. To use a rather obscure example but one that's occudred to me a lot - compare the BBC arts documentaries that are produced now to the likes of Civilization and Ways of Seeing. Visually they're high definition, the camera work is more fluent, etc. But because of this emphasis on flashy visuals, coupled with a decline in attention spans (maybe), there's a lot less actual content and analysis going on.