Thing is, Mickey is much more than a "cartoon mouse", and he definitely meant more than that to Walt.
“Mickey Mouse is, to me, a symbol of independence. He was a means to an end. He popped out of my mind onto a drawing pad 20 years ago on a train ride from Manhattan to Hollywood at a time when business fortunes of my brother Roy and myself were at lowest ebb and disaster seemed right around the corner. Born of necessity, the little fellow literally freed us of immediate worry. He provided the means for expanding our organization to its present dimensions and for extending the medium cartoon animation towards new entertainment levels. He spelled production liberation for us.” – Walt Disney
“The life and ventures of Mickey Mouse have been closely bound up with my own personal and professional life. It is understandable that I should have sentimental attachment for the little personage who played so big a part in the course of Disney Productions and has been so happily accepted as an amusing friend wherever films are shown around the world. He still speaks for me and I still speak for him.” – Walt Disney