With written material, like a book, it does go into public domain after a period of time, but I thought it was 27 years, though I'm not sure about that. But all one has to do is re-register the copyright to keep that from happening. The heirs of famous writers--like John Steinbeck, for instance--do it as a matter of course. It think in some cases, where there is no well defined estate, the publisher can keep the copyright current.

             It seems to me, however, that really famous material should go into public domain at some point in time--Charles Dickens comes to mind.