So should Runaway Railway be replaced because several Mickey cartoons were racist? If we are using “problematic source material” as the reason for change, logic would apply there too.
Why is Mickey safe, but Brer Rabbit isn’t? Is it simply because “he’s Mickey”? To which, from a business perspective, that makes sense. But from a moral perspective, it’s still wrong, and makes this whole thing seem rather half hearted.
Disney already doesn't show a number of Mickey cartoons due to very racially insensitive material in them (e.g. the one where he paints himself up in blackface for a performance). Warner Bros. has done similarly for various depictions of blackface in Bugs Bunny cartoons. And yeah, Mickey and most older cartoons had a lot about their designs based off of minstrelsy, as well.
Mickey's origins, however, were not in propagating a racial system, despite his design elements, and he's a character with over 90 years of history behind him at this point who has been used and reused in an enormous multitude of ways and styles; he is not defined, in the public mind, by the worst of what was in his history, because he's been changed around and presented in various ways by different animators, voice actors, writers, etc. over the years.
Ironically, the Uncle Remus stories were initially compiled with what seemed like good intentions in mind, but we're referring here less to the Brer Rabbit of the old stories and more to the characters of, specifically, Disney's version of Song of the South. Due to that movie being "vaulted" so often, it's been very difficult to grant Brers Rabbit, Fox, and Bear much life beyond their use in a film that is, let's face it, pretty darn issue-ridden. Fair or not, they're perceived by the public that knows them as characters from a film that Disney itself isn't comfortable promoting to this day, while characters like Mickey and Bugs are not trapped being defined by racial caricatures they might have performed in the past.
Again, I'll still say I'm not sure I agree with making this change, overall, but I don't think it's an entirely fair comparison to say "if Brer Rabbit has to go, then so does Mickey!" It's taking an all-or-nothing, "there can be only two sides/outcomes" stance on something with much more nuance and complexity to it.