I think the suggestion needs more thought, but I don't necessarily disagree with the idea of making my exit before the dementia really sets in. I've directly cared for two family members who suffered from it (collective 5 years of my life, which I'd like to think gives me a pretty good view of what the disease can actually do to people) and I decided for myself that I'd rather be quietly killed than put my loved ones through what I went through as a caretaker. While also trying to work a full-time job and maintain my own sanity while I watched people I'd known all my life be destroyed, becoming tortured versions of themselves like something out of a body-snatchers horror film.
We, the loved ones, made the decisions to keep them going and I wonder how fair that was to them. We tend to not want to let people go, choosing to sacrifice quality of life for the sufferer and those around them for, what, a few fleeting moments of possible clarity? The opportunity to say goodbye to someone who may or may not even understand what is happening?
The events I went through with my family hurt us in ways that will not likely ever heal, despite effort on at least a few of our part, and it did leave me wondering if I would put my son or wife through that should something similar ever happen to me. I decided against it, seeing as I am at the age where these are very real possibilities. In the US, we have DNRs ("do not resuscitate") and living wills that offer prior directives, but something like assisted suicide is not allowed here unless some very extreme circumstances are met, because insurance companies and hospitals make more money from suffering people than dead ones. I'm a strong advocate of the right to die, but it is a decision that needs to be made some extensive consideration and documentation before one actually needs it.