While I feel for you, I would like you not to do something that cause cryonics to be banned.
What Alzheimer's does seems to be unknown, but at least it the late stages it seems implausible that it wouldn't destroy information, it shrinks the support too much.
The stage at which you have to maybe-die seems less determined by how much of your personality has been destroyed by the disease, and more by how much people stop deeming you competent for choosing it. Moderate dementia doesn't seem to be anywhere near enough to kill you but can definitely be used against your choice to e.g. starve.
I will keep the whole "don't get it banned for other people" thing in mind should this ever actually come up, but as I can't predict ultimate outcomes I can't make any promises.
Choosing not to eat or drink doesn't seem likely to provoke that response, especially if I only explain it as "I don't want to live through [whatever]".
I would make such a choice long before I was considered legally incompetent; the Russian roulette of "any day a fond memory could disappear" is to horrible for me; unlike other people in this thread, I consider my memories a central part of who I am.
So being signed up for cryonics shifts my views on life and death, as might be expected.
In particular, it focuses my views of success on the preservation of my brain (everything else too, just in case, but especially the brain). This means, obviously, not just the lump of meat but also the information within it.
If I'm suffering a degenerative disease to that meat or its information, I'm going to want to cryocide to preserve the information (and the idea of living through slow brain death doesn't thrill me regardless).
What I don't know is: given the current state of science, what sorts of things do I need to be worried about?
In particular, I'm wondering about Alzheimer's; does it appear to be damage to the information, or to the retrieval mechanism?
But any other such diseases interest me in this context.
Thanks!
-Robin