If that's the case then why not a mentally retarded human being in an institution if that's what the family desires? Their quality of life is in many cases the same as Terri's was. What's the difference? Why was her case special or different?
I don't know the answers to your questions. But I do wonder what is appropriate even in those situations.
The difference regarding Terri Schiavo is that she was immobile. She was unable to chew or swallow. She was unable to communicate.
I dont know how similar her situation was (or dissimilar) to the mentally retarded human beings you refer to in your post.