I -- I understand, I think. My mind has always been the most important thing to me and dementia is my number one fear (along with any kind of brain injury). If he comes to a place where he's ready, then the choice should be his. Slate had an article on the topic.

Or in the words of Jonathan Franzen --

After we’d kissed him goodbye and signed the forms that authorized the brain autopsy, after we’d driven through flooding streets, my mother sat down in our kitchen and uncharacteristically accepted my offer of undiluted Jack Daniel’s. “I see now,” she said, “that when you’re dead, you’re really dead.” This was true enough. But, in the slow-motion way of Alzheimer’s, my father wasn’t much deader now than he’d been two hours or two weeks or two months ago. We’d simply lost the last parts out of which we could fashion a living whole. There would be no new memories of him. The only stories we could tell now were the ones we already had.

Alzheimer's is a wrenching experience for both the afflicted and all those close to him. I hope Pratchett's fandom lets him make the choice that he feels is right for him. Though I'm imagining a lot of "SAVE TERRY PRATCHETT" Twitter petitions. :|