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.
If you can't learn to do it well, learn to enjoy doing it badly.