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.
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.