Well, consider that the vast majority of older films are no longer really watched by anyone. Silent films, for instance, and most black and white ones. Art is art because it is admired generation after generation. If films cease to be interesting as the technology of film advances, then they cannot really be art, and must instead be viewed primarily as being of interest for what they can tell us about the historical period in which they were produced.
Or, if you wanted to argue against the statement, you could point out that films such as "A Wonderful Life," "A Christmas Carol," and "Citizen Kane," continue to air half-a-century after their initial release, proving that at least some films can transcend their historical context and manifest universally interesting characteristics that elevate them to a form of art.
Whichever approach you decide to take, this should at least get you started.
