I may have expressed myself poorly, and if so I'm sorry.
Cognitive neurodegenerative diseases like Alzheimer's fall into a class of diseases. It excludes people who are brain-damaged, whether as a result of cranial trauma, strokes, or brain hemorrhages. That's an entirely different category. These patients will either have already given their advanced directives, which is nevertheless relatively rare, or not done so. If they have not, then the decision burden falls to the care team and the family, as is currently the case with respect to aggressive therapy.
Cognitive neurodegenerative diseases are an entirely different category.