I believe you can make provision that, should you arrive at a state where it is medically assessed that you have no quality of life and no prospect of recovery, all interventions, bar keeping you as comfortable as possible, should be withdrawn. For instance, no antibiotics would be used to combat pneumonia or other infections.
I can see your point petallus about those Alzheimer sufferers whose lives become a living nightmare, and most of us would dread ending up in this way or having to see it happen to anyone we know. However, this comment by KH seemed almost contemptuous of people with dementia rather than being a compassionate response to suffering.
My understanding is that to relieve pain increasing amounts of drugs can be administered, even if that may cause or hasten death. But drugs cannot be administered with the intention of causing death. Perhaps those who have been in the nursing or medical profession can clarify.
I agree with you Flickety that the contribution of many older people is often overlooked. However, even if someone can't or won't make much of a contribution to society, they should not be consigned to the scrapheap. I'm sure you don't mean that, but we are increasingly being encouraged to look at people in terms of their value to society rather than their intrinsic value as human beings.
It also occurred to me that once there is an assumption that a person with dementia would really be better off dead, couldn't this idea then be extended to other people who might be considered to be in torment or to have no quality of life - those with profound brain damage/learning difficulties, intractable psychiatric illnesses, etc. etc.