I’m honestly sure you don’t mean “retarded” as a personal attack against people with developmental disabilities.
There’s also a very long history of people using that exact same word to attack them. Many people who make arguments like yours, that they should still get to use the word, tend to also be prejudiced against these people and treat them poorly. Maybe you’re not like that, but using that word does make you look a lot more like these people.
You making an argument to try to keep using the word, even though it hurts people, essentially tells them “I will not change one word in my vocabulary to accommodate your feelings and history of being hurt by this word.” Your choice on if you want to change your words to in an effort to get people to interpret you in a way that better matches your intent, or if you’d like to stick to your guns even if it means lots of people will get hurt. After all, people are free to interpret words however they wish, including in ways that you don’t intend them to and ways you don’t mean. I don’t think it’s unreasonable for people affected by this word to not want you in their communities if you want to keep using that word, at least if they didn’t promise you absolute freedom of speech.