There’s a qualitative difference between an able bodied person who just isn’t very bright or doesn’t put a lot of thought into things VS. a person with a mental disability. Idiot, stupid, dumb, etc. in the modern context are used to describe the former, while the r-word is used to describe the latter
People getting offended are just dummies because…? I don’t remember the kids who needed special assistance in elementary school being called cretin or imbecile when being harassed but i do remember the r slur.
Not up to you what or why people get offended but this one specifically has real passion behind it for a lot of people.
Lots of ableist language is acceptable, therefore the word with arguably the most contemporary relevance that offended the most people should also be acceptable.
Is this what you’re saying or, no? If it is, it’s nonsensical unless you place the funny phonics over the mental well-being of a disadvantaged group