I would imagine, at some point, the compromise will happen that the old works are in the public domain, but newer trademark laws protect the characters from being used in other people's new works. That is their biggest fear anyway.
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.