My father currently has Alzheimer and moved to a service home a month ago in Sweden. He is having a good time at the home, and keeps saying that he doesn't understand what he is doing there, as he doesn't feel sick. It is the family that is suffering from the change in him, not my father. Quite the opposite, if you sit with him showing him pictures he is very excited. He likes taking walks, drinking coffee, talking, playing the piano. And it is not totally ungratifying being around him, as you can tell the same joke twice and he will not remember it the second time.
Thus, Alzheimer is in my opinion not a candidate for assisted suicide, as the patient does not suffer from the disease. True, it is not the same personality as he had in his "sane" days, and he needs constant assistance. But saying that a life is less worth because you are only satisfying basic instincts, is egoistic on the side of the family, who wants dear loved one stay the same as they remember him.
Terry Prachett who is today saying that he doesn't want to become the demented Terry Prachett does not represent the wishes of the demented Terry Prachett.