John Bergmayer has published "Let's Make It Easier to Expand the Public Domain" in Copyright Reform.
Here's an excerpt:
The fact that a license is "perpetual" doesn't require the copyright holder to keep offering the license; it just means the license, once granted, can't be revoked.
Except it can be. Copyright termination means that any license, including a perpetual public license, can be revoked. This means, for example, that contributors to projects like Wikipedia (where an original contributor continues to own the copyright to her work, but licenses that copyright under a liberal license) can revoke that license. It also means that people who transfer actual ownership of their copyrights to stewards like the Free Software Foundation can claw back that ownership.