There are few people alive today who would remember when steamboat Willy was released. Culture is currently tied up in copyright long enough that things will mostly be forgotten long before they enter the public domain. Effectively lost. This is surely not desirable.
If you asked any random person on the street, chances are they know who Mickey Mouse is. It doesn’t really matter what “content” is made with him. Disney can just keep pumping out cheap CGI kids shows for brand recognition alone and Mickey would last for at least another generation.
I think Disney stopped developing a lot of new content using Mickey because they knew he was going into the public domain and the ROI would not be as good as using new characters.