Wait until they realize that every single fucking insult in existence is equally "able-ist", by definition, or otherwise offensive in some way. Is the expectation that we slowly disallow insults in culture? No, it's going to be arbitrary whitelisting. Stupid (there I go being ableist).
A change like this doesn't work as intended without fundamentally changing the narrative. If a character was insulting someone, they should still be insulting someone. Want the story to be different? Write a different fucking book.
The current equilibrium seems to be that we're on a sort of vulgar treadmill. The process goes as follows:
1. Decide that previously formal/medical/technical word like "idiot" has developed too many colloquial negative connotations.
2. Invent new formal term, like "mentally retarded", and tell people that they have to use that instead, because the old term is now considered offensive.
3. Because connotation emerges from denotation, the new term develops precisely the same colloquial negative connotations.
4. Rinse and repeat.