I thought the whole reason for the extensions was not Mickey Mouse (he is covered under trademarks anyway), but Winnie the Pooh. Unlike Steamboat Willy Winnie is still making billions every year for Disney [0].
That is not what wiki has to say on the matter (the Winnie part, not the Disney lobbying). This is why I included the reference because I have heard this explanation before.
0. https://en.wikipedia.org/wiki/Winnie-the-Pooh