Yes, difficult one, but PWD don't have the opportunity to make a choice, do they, because they have, by the very nature of their illness, lost the ability to process and understand information. What you may have said when you are younger and healthier may not apply as death gets nearer - people do change their minds.
Both my parents say they want to die a lot of the time, particularly if they are in pain or depressed, and quite honestly if I could help them in this regard I would be seriously tempted to do so, as in my judgement they have little quality of life and they are never going to get any better than they are now, only worse. I do think people with terminal illnesses who wish to end their life rather than continue and die a lingering, painful death should be able to do so easily and, hopefully painlessly, but with PWD that is a whole different kettle of fish. I'm glad in a way I don't have to chance to make that choice regarding my parents, but I do pray they will go quickly, soon, and close together.