Considering the public domain is up to 1924 and Steamboat Willie is from 1928, Disney is certainly going to start pushing for an extension soon if they haven't already started lobbying for specific legislation on the issue.
I bet they try something different this time than just an extension... maybe some sort of "brand icon" exemption that exempts any character who is strongly associated with a brand from copyright expiration. They probably sense that continuous extension isn't going to work forever.
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.
My intuition tells me that those of us who care about freedom will find a way to seep through the cracks of any wall attempting to destroy it. I agree the current atmosphere is bleak, but let's not give up so easily.
I just built the Steamboat Willie LEGO set. Was wondering what prompted that particular curiosity; copyright extension leveraging seems a plausible motivation.
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.
As long as we are watching Disney won't try. Votes are more powerful than money in politics when the voters are paying attention. Thus so long as we are paying attention Disney won't try anything. If we ignore the issue though...
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.