I agree! Although the most popular movies will be preserved, so much other stuff will disappear, never to return. It's already happening.
If it weren't for these long copyright laws, people could openly archive and share these cultural artifacts so that they will be available to future generations.
Incidentally, this is why I think electronic-only, DRM-encumbered releases of books, music, and videos are vastly inferior to physical copies of the same. Physical media (well books and vinyl records anyway) will outlast technologies and whims of history, and can be digitized hundreds of years from now.