When a songwriter and music publisher sign a deal they decide how long that contract will last. And that the publishing deal will specify the duration a music publisher will administer songs on behalf of the songwriter. Smart musicians who have a broad understanding of music publishing will not allow a music publisher to be a copyright holder forever. Because in such a scenario the music publisher will earn money from that intellectual property until well... forever (ad infinitum).
For example, a songwriter may enter a deal with a music publisher for a number of years. And when the time comes to pass, either the songwriter will renew that publishing deal or get their music publishing copyrights back. Another way a songwriter may...