Since you've decided to push back, I hope you'll do me the courtesy of reading what I'm about to write and think about it carefully. Initially, your comment made me feel angry. So I read it multiple times, took a breath, calmed down, and now I'll try to respond in a productive way. This is kind of long, but please stick with me. Think carefully. Do you know and love anyone with a disability? Would you denigrate them to their face; call them "retarded"?
For me, this isn't a question of "political correctness." It's about recognizing the human dignity of every living person, whatever their level of physical or intellectual ability or disability. Calling someone a "retard" carries a negative accusation about people with disabilities. You are cashing in on someone else's disability in order to tear down or hurt someone else. You seem to be saying that it should be an insult to be called a "retard," and that it should be shameful. That the worth of someone with a disability is so low that you can use their disability as a label to shame someone. I think that's terrible.
I actually agree with one thing you said: You're right that using such language demonstrates a person's prejudice. In some ways, it's actually good to know that someone is a racist, bigot, or ableist. But I don't want to leave it at that, because prejudice itself is the real problem. Language reflects a prejudiced mindset. When I ask someone not to use a word like "retard," I'm implicitly inviting them to start reevaluating their prejudices. They can begin changing their mindset by paying more attention to their language. I personally went through that process. I'd be happy to tell you the story if you're interested.
You might be worried that my request not to use certain words will result in a sheltered society where people are too weak to withstand difficulties. As though we need racial, ethnic, or gender- or ability-based insults in order to become stronger people. I believe it takes more strength to revise our language. I'm all for helping people develop grit, and I try to develop grit and resilience in myself and my kids. They'll need it, because there are plenty of people who will insist on saying things like "retard" because they don't want to be fake and dishonest. I think the "real you" shouldn't feel good about demeaning the dignity of people with disabilities. If that's the "real you," then I invite you to change. Then you won't have to worry about being fake and dishonest, because the real you will understand the value inherent in human life in a way that lifts everyone.