As I said, I come from the research community. It is easy for us to quantify loss of autonomy using research tools that can be simplified, and literally determine a point on a scale from 1 to 5, or from 0 to 30, and say the person has reached that threshold. From that point on, the family should be able to step in and activate the person's wishes.
That is my idea of how it should function. I particularly like the way they do things in the Netherlands, but I might add in a bit more involvement on the part of the family, so it is a little bit less completely managed by the attending physician.