We are already seeing this in the real world, where Disney cartoons are public domain, but the characters, having been used in consecutive works, cannot be used by anyone other than them.

This is incorrect. When a Disney cartoon becomes public domain everything in it is also public domain, including the characters as used in that cartoon. The most famous example of this will happen on January 1 when the first Mickey Mouse cartoons go into the public domain, and so will that version of Mickey Mouse. You can read more about what that means for Mickey, and for Disney, in this post by the Center for the Study of the Public Domain at Duke Law.

Nobody’s legacy is “tarnished” or otherwise damaged by things other people create. The original is still there, while new things get to express their take on the characters and/or the rest of the material. Derivative works add to the sum total of culture, they don’t subtract from it, and the Public Domain denotes the part of culture we all own together and can develop new works about freely. The freedom to do so is a good thing for everyone including cultural creators (who get to enrich their own work using our shared property) and consumers (who get more stuff they might enjoy, and if they don’t the original is still there regardless) and everyone wins. Your scenario would make nothing better for anyone.