So you don't have copyright but yet you still have copyright rights on the characters? That would be a weird legal limbo, let's not go there.

The even simpler compromise is that older copyrights that do not have a current registration maintained fall into the public domain. Registrations must be refiled every few years to stay current.

This compromise would fit current interpretations of copyright, allow Steamboat Willie to continue to be effectively permanently copyrighted, without constantly rewriting copyright law. It would also allow the vast bulk of existing old copyrights to become public domain.

It is a compromise where each side gets most of what it cares about, and we don't need to have progressively worse laws passed.

Personally, I believe that any system allowing for indefinite copyright protection (even a system like the current one, that involves constantly changing the system to extend protection) is harmful in the long run, but your compromise is definitely far superior to the status quo, and would probably make for a pretty good bridge from now to future, far more sane copyright law.