There is no copyright on the character itself. There's a copyright on each piece of media the character has been in, like Steamboat Willie, but there is no copyright on Mickey Mouse. The character is trademarked, but
not copyrighted.
The words "copyright" and "trademark" are different, and mean different things. You might know this and be fuzzing them for the sake of simplification, but if so, you've over-simplified. They are different, and treating them as the same is wrong and leads to confusion.
The copyright on Steamboat Willie will expire, allowing royalty-free use and copying of that movie, including Mickey Mouse. At that point, that version of Mickey Mouse will be free to us--BUT the ongoing trademark on the character will mean people should still stay away.