I think that there are a lot of good reasons not to use the word "retard". And there aren't many good reasons to use it. I know of plenty of alternatives.

Agree completely.

But how often do people use the word cretin?

Most people have no clue what that word means or how it originated. I certainly don’t use “cretin,” since I have no use for disparaging someone as mentally and physically crippled. Maybe that’s your point, that properly understanding the genesis of some term can undermine your desire to use it? And you’re right. Cretinism, the disease, makes me really sad, as does the fact that assholes chose to turn it into a pejorative. So maybe that has something to do with my unwillingness to ever use the word.

In my mind, “retard” was more of a vague diagnosis of mental slowness, so it makes it less real as an actual medical condition. Like when you say “retard” I think “Republican.” Those are the people who need diagnosing. Still, I’m less willing to use the r-word than alternatives like “idiot” whose meaning is totally divorced in my imagination from any origin story.

After all, once you use a word (a bunch of sounds) to mean something long enough, it eventually makes no difference what the word used to mean. That said, I can see your point. The cretin example is a good one. Very persuasive.