Because Mickey Mouse is an icon, a mascot, a trademark, and one that is in active use in new media. So there is some sense in companies protecting that asset. Which is what is happening if these types of characters are covered under trademark law (I wasn't sure if that was the case when I made my earlier post).
I might dislike Disney as an org but I also don't think it's fair to say it's "creative" for other people to use that asset but not "creative" for Disney to reuse it. You're setting different standards for different people.
> Copyright's only base for existence is encouraging creating creative works after all.
Copyright is an English invention and the purpose of it has evolved multiple times over the years. Originally it was intended to prevent scandalous pamphlets rather than promoting creative works. Later it was used like patents where authors could recover the costs of publishing before those stories became free for the masses. It wasn't created to encourage creating creative works because people used to do that long before the printing press, eg by passing stories down by word of mouth.
> Note that you can't use elements still under copyright, e.g. his modern appearance won't be public domain once Steamboat Willie enters it.
Indeed. But I was responding to the OPs point about how they believed copyright should behave rather than how it currently does.