I read—within recent memory—"Consciousness and the Brain: Deciphering How the Brain Codes Our Thoughts" by Stanislas Dehaene and was very surprised by just how much consciousness research is being done and how much has been learned. It was really very interesting and I think this research has the potential to make the trade offs between suffering and possible recovery much easier to answer. (This isn't to disagree with you, just to express my surprise that, although I agree with you, researchers actually know/suspect a lot more than I thought they did.)
I think the degree of suffering and misery that comes with death due to attitudes that are imposed on us by our Puritan Christian Heritage is a serious tragedy.
I can't think of a reasonable argument against my position. My body, my life, my decision. And I have the right to make that decision while I'm mentally competent, and have it respected if/when I'm not.
How, if you don't mind sharing? Something like a passphrase shared with a trusted loved one?
Here's one scenario that is feasible today. Imagine an IV drip controlled by a computer. If I enter the correct password when prompted (say once every 24 hours), nothing happens. However if I am prompted repeatedly to enter the password and I do not do so, the IV drip is actuated. The drip contains an end-of-life cocktail.
This puts the onus on me to make an affirmative choice each day to continue to live. If I am unwilling or unable to enter the passcode, that indicates either 1) my condition has deteriorated to the point where I am cognitively or physically so disabled that I cannot remember and physically enter the password, or 2) I am making a conscious choice to end my life.
There is no action to be taken by anyone to cause my death; the decision rests entirely with me, and is effectuated by a machine, not a person.
Or (3) hardware or software defect exists and has been triggered. Fail-deadly systems are usually disfavored for very sound reasons.
Now, I suppose in this situation you could be convinced enough that you are going to want to die so imminently than the risk/cost of an accidental early trigger is acceptable, though.
I don't think in the US or Canada there is any way to legally end your life at that point. Maybe nowhere in the world. You would need to end it while still competent. That would be a difficult line to tread because you don't want to forgo good days and with dementia the good days are the ones where you are competent. By the time the days are bad you can no longer competently end your life.
In an ideal world you could specify some kind of ego-distance from yourself where a painless shutdown would be triggered. But of course we are far from having the ability to measure or even formally describe this concept.