You could combine it with annual payments. Free copyright for the first 20 years, then annual payments until death of author (person or other legal entity). Disney will still like it, because it will be less money than lobbying, but it will allow older now-unprofitable works to enter the public domain.
Or outright require that the corporation in question shows continuing profit from the work. If they're not profiting, then they're not using it, or are outright misusing it, so everyone else should be allowed a go at it.
Or allow for this transition of authorship to only happen when the corporation itself is inextricably tied with the work in question and would be severely damaged by its passing in the public domain (yes, this is in effect "do you have enough lawyer money" - kind of the same principle as above). Maybe require that the corporation was funded no more than delta(Disney creates Mickey Mouse, Disney creates the Disney corporation) + 1 years after the work in question. This will prevent for example book publishers getting perpetual rights to a book.
However it is formulated, I do think we can afford to cede certain corporate icons to their respective corporations in exchange of saner laws for the common cases.