The argument about directors' copyright seems to be based on the fear that one creator's rights will somehow diminish others. But if stage direction is copyrightable, it is an entirely separate work from the playwright's text, just as choreography is an entirely separate work from the composer's music. Copyright in one doesn't affect the other. Or by analogy to derivative works, the copyright in the derivative work (any adaptation of a work into a different form) does not diminish or affect the copyright of the underlying work at all, and what is copyrightable about the derivative work is only the new material created.