The reason copyright is not supposed to be forever, is because everyone builds on previous works. Nothing is 100 percent original. Maybe not even 20 percent original, if you really start deconstructing something and rip the most fundamental ideas out of the "new" creation.

Plus, if you then again allow others to build on the more recently "invented" stuff, then you get even more ideas and creations, as a society, which is why there's this "agreement" that copyright works should go into public domain. But corporations, who don't invent anything themselves, want to be able to exploit those works forever.

This is why I support Sweden's Pirate Party's idea [1] to limit copyright to 5 years, and then allow creators to extend it every 5 years up to 20 years, for a fee each time, because then you wouldn't end up with problems like these, where the vast majority of works become abandonware that nobody can use for almost a century to at least remix them:

http://www.theatlantic.com/technology/archive/2013/07/the-ho...

Watch these videos, and then hopefully it will make it more clear:

http://everythingisaremix.info/watch-the-series/

[1] - http://copyrightreform.eu/