The thing is, they can keep him as a trademark even if he is public domain. This is how the Edgar Rice Burroughs Estate keeps Tarzan and John Carter, for example.
Maybe it should just be increasingly costly to maintain an old copyright, so a few high-value copyrights like Mickey Mouse could be maintained while much more literature simply passes into the public domain. Or perhaps film copyrights could be separated from print copyrights.
Or maybe we could just not have de facto perpetual copyright.
But trademark is restricted by trade classifications, and must be monitored and defended vigorously by the trademark-holder.
If you sell construction equipment, you can have a Tarzan brand of earth-mover. If you sell space tourism, you can have a John Carter brand of travel package. In order to lawyer-smack you, the estate would have to show the court that they were already using the trademark in that line of business before you.
So you could call anything that Disney does not already sell a "Mickey Mouse". Think of something NSFW. When they go to C&D you, you can demand to see proof that they are actually in that line of business. They either successfully defend the trademark and get a PR disaster or you get to use the trademark for your trade class.
I think it's a little more complicated than that. The primary goal of trademark is to prevent consumer confusion. Mickey Mouse is so closely associated with Disney and used with such a dizzying array of products that calling something Mickey Mouse, even something NSFW that Disney wouldn't want to make themselves, seems likely enough to cause confusion.
It's not, really. Disney does not sell "adult massagers", and everybody knows it, so if I want to make one and call it the "Mickey Mouse", they do not have grounds for a trademark infringement action. They do not sell that type of product.
Once Steamboat Willie is in the public domain, I can even put a picture of Walt's original Mickey character on the box.
Of course, the profit margins on a product that does not set fire to the "sue me now" beacons are usually going to be higher. If you are not already aware, the international intellectual property protections are more beneficial to people and companies that have more money to spend on invoking them.
So trademark holders have to monitor and vigorously defend a trademark that is more limited than copyright. What's wrong with that, for an 80-year-old work?
Is this really a worse system than one in which a huge volume of literature and research from the 1930s to the 1960s is locked into an out-of-print limbo, not "valuable" enough to republish and yet virtually inaccessible outside of research libraries?
The fatal flaw of Trademark is in the line of business. Disney would be liable for anything new. That makes the millions of dollars to continue to flow to protect new items coming into public domain.
What if we could have the multi-million dollar Copyright license. It cost MILLIONS for them to keep things out of Public Domain for 25+ years, therefore things not valuable fall to public domain
Maybe it should just be increasingly costly to maintain an old copyright, so a few high-value copyrights like Mickey Mouse could be maintained while much more literature simply passes into the public domain. Or perhaps film copyrights could be separated from print copyrights.
Or maybe we could just not have de facto perpetual copyright.