I'd argue that if an art piece is famous enough, then it ought to become "public". In the sense that, yes, the artist made it and it became famous, but once it reaches a certain scale of fame, it means that the public itself made the work famous. So it's a kind of co-creation (eg for instance the fact that people were writing fan-fiction about Harry Potter or whatever, contributing to the fame of the book, etc). So in this case, after it has reached a sufficient level of fame, it should become public work