I understand how you see it that "doing nothing" does sound like sticking your head in the sand. Although my husb has Alzheimer's he was still able to understand enough to make an informed decision. He had filled out an advanced directive several years before and his decision after diagnosis was the same as it was prior.
Alzheimer's is such a terriable way to die, as cancer can be also. Had the choice been up to me I would have made the same choice he did. There are also other health issues at play also. Life is precious but quality of life needed to be a consideration for us as well.
Had he opted for surgery it would have required removal of half of his face including an eye and that would only slow the process. Also the subsequent treatments would have required three weekly 100 mile trips for eight weeks and physically that would have been too much for him also. For all we have family in that area, putting him in unfamilar surroundings would play havoc with his alzheimer's. His age is also a factor. I feel he made the correct decision for his situation.