Several things can be true at once.
Splash Mountain is indeed a charming masterpiece, loved by many (myself include).
Few people have any real knowledge of the IP to which it is tied. Many, and perhaps even a majority, of those riding it may have no idea whatsoever of its source material.
However, the IP in question happens to be a film that has been regarded as controversial from the moment of its inception. Since 1946, long before most of us were born, criticism has been levelled at Song of the South for its portrayal of plantation life. The debate started over half a century before the fabled "woke mob" can be accused of getting involved.
What this means is that Splash Mountain, no matter how charming or beloved it may be, and regardless of how little-known Song of the South is today, has an indelible connection to something long perceived as problematic. That connection was previously thought of as tenuous enough not to matter; now, Disney considers it to be a liability.
I'm not sure why those who object to the retheme aren't at least able to acknowledge the chain of circumstances that have brought us here. No amount of obfuscation or denial can change the fact that Splash Mountain would never have existed in its current form had it not been for Song of the South. Its relationship to the film cannot be pretended away; it's baked into the ride itself.