Absolutely, it should be available, especially to terminally ill, or constantly in pain, people. As per usual, it is mainly religion which holds society back from allowing this procedure to be validated. The dilemma is that one has to choose euthanasia, which suggests being of sound mind. Therefore, people with dementia etc could not avail of it. For example, you could have an uncle who in his day was the life and soul of the party and now he sits all day in a chair, dribbling, and if you went to see him, he knows not who you are.
He may have to be fed liquidised food through a straw and have his bum wiped after each toilet visit. One would assume he does not enjoy his existence anymore but it would be hard to justify taking away his life to society.