The logo for Disney Animation before their movies is a clip of a few seconds of Steamboat Willie. I expect them to lean even more heavily on trademark protection once the copyright expires.

Besides, it's not like the copyright on the old theatrical shorts is that valuable. The real interesting stuff will be in the 2030s when the feature-length films start entering the public domain...of course by that point you probably won't be able to find any non-DRM video, if current trends continue.

That would probably be a violation of Berne Convention because brands are national constructs and require registration and convention requires copyright to exists independently of national borders and without registration.

Disney recently released some special edition gold fish crackers with the Steamboat Willie design. They are pushing for something.

That'd fall under trademark law, no?

Disney themselves according to shareholder reports also believe that Trademark law is in a strong enough position today that they aren't officially concerned about further copyright extensions when they can use the power of Trademark lawsuits to protect their IP. The copyright term is just one moat in their strategy, and they may indeed feel (as they've told shareholders) like they've made that copyright moat big enough they can focus on the next moat.