Try telling that to my boss, who has an autistic child. I held your opinion until I stuffed my foot in my mouth in a meeting. Now I don't use that word.

There it is.

"When you omit courtesy you're throwing sand in the gears of a machine that doesn't work too well in the first place."

~Heinlein (I'm paraphrasing.)

There's no glory in being a boor, and no shame in being courteous.

None of this is news: https://en.wikipedia.org/wiki/Etiquette#History

    A fool never learns,
    A man learns from his mistakes,
    A wise man learns from the mistakes of others.

Wait until a derogatory word being used effects you directly. It's really easy to just not use words flippantly that you know some people have an issue with. You are clearly aware of how it can be offensive and for a lot of people your definition is still just a callback to people being unkind about mental illness. History matters too and if you choose to ignore that and use it anyway you're just being unnecessarily inconsiderate.