This has to do with the issue Ms. Pastine referred to concerning advance declarations. The Carter criteria describe enduring suffering intolerable to the individual in the circumstances of his or her condition. I'm thinking of Alzheimer's disease and the issue of dementia that Wanda Morris of Dying With Dignity referred to. By definition, the person would not be suffering or would not have the kind of enduring suffering, perhaps, if they were unaware that they were in pain. They're not in pain if they have dementia at the end of life. They might provide an advance declaration for that.
I take your point about ceilings and floors, but I wonder if it's open to us, in faithfully applying the Carter criteria, to address dementia as you've described.