Perpetual copyright on copyrights as they are today would cause a great amount of work to be lost due to the owners greed and stupidity.
As per my suggestion on licencing a work could become free-use much sooner than the copyright period expires. Anything with no units shipped would mean its a free license.
As I've said elsewhere, and originally, I'm in favour of 20 years and extensions for derivative works, and perhaps then only on the right to produce derivatives and on no other rights like reproduction.