I think it's likely that Alzheimers does irreparable information damage. If I'm diagnosed, suicide would certainly be my choice before it begins to severely impair my cognition. There are ways to kill yourself that resemble natural death and leave only minimal evidence of any kind (I am thinking specifically of nitrogen asphyxiation). You'd need co-conspirators and some careful timing, but I think it might be achievable.

That said, I do think there's a huge demand for a cryonics organization that gets rid of ridiculous, barbaric things like death by dehydration as a legal route to suicide. Perhaps something in international waters, or a nation more friendly to medical suicide?