That's going to be 80% of us someday. That's grounds to euthanize him? Keep him comfortable and let him die a natural death from the dementia or the sepsis if he really is dying, but why take away his water and food? That's euthanasia, that's not "letting him die."
Natural death didn’t used to include IV’s and medical intervention for people who have no hope for recovery. That makes it different from Terri Schiavo.
My father died of an inoperable Glioblastoma IV. It had taken over most of his frontal lobe and rendered him blind and unable to speak. He was a vibrant man, a triathlete at 77 and he had a living will. The cancer made him unable to eat or drink. He would not wanted to live that way or prolong his life in the hell that tumor wrecked on him.
Everyone’s situation is different. Broad brushes do no one favors.