[snowwrestler]

The copyright on Steamboat Willie only protects Steamboat Willie. Copyright is specific to the work it protects; it does not provide general protection of the characters in the work.

Mickey Mouse the character is protected by trademark. Unlike copyright, there is no time limit on trademarks. As long as they remain in active use and defended by the mark holder, they remain protected. To be clear: Steamboat Willie could enter the public domain tomorrow and it would not affect Mickey's trademark status.