It's not about Mickey specifically, but rather about what it represents.
The entry of new works into the commons was shut down for a long time, with Disney keeping Mickey under copyright being viewed as the main driver for the lobbying that's kept it closed. We're coming off of two sequential 20-year extensions, and got our first works entering the public domain in the US in 2019, at which point there was a clear deadline for Disney to either get copyright extended again or cede the point.
So Mickey starting to creep into the public domain means the commons probably won't be closed again, since nobody else has as much interest in pushing that as Disney did.
Because culture is built on itself. Almost everything is built, remixed, rewritten at some level. A good chunk of the Disney classics are taken from older stories, fairy tales, etc that were by then public domain.
CS Lewis’ last novel was inspired by an ancient myth, and it wasn’t a quick rehash. It was reworked and retold with a new voice. Perhaps Steamboat Willie will be the same!
