Copyright hampers rapid repackaging of ideas, which is a major vector for innovation. Wagner's Ring Cycle plot was a refactor - a repackaging of existing stories and ideas. Much of Handel's _Israel in Egypt_ (and many other works) is a repackaging of earlier tunes. Bach's Goldberg variations remixes then-popular tunes.
Consider cross-pollination. Nintendo wanted incidental background music for tetris. They found some tunes in the public domain, and wrote some themselves. Only possible when there's a public domain. The game isn't about the music, but the fact that they could mix good music in made it better.