What I’m saying is that the philosophical issue you raise is irrelevant in the real world. If I decide, while I’m still thinking clearly, that I want to die when my mental abilities reach a certain point of decline, whether or not I am thought by other people to be “happy”, rather than waiting for agony and mental anguish, then that should be honored. Your evaluation of a demented person’s “happiness” is irrelevant.
If the state of dementia was reversible, and if there was any hope of a full recovery, and a resumption of a healthy, thinking life again, you’d have a point. But that’s not how this ever works. At the point in life we’re discussing, there’s no road back, and death is inevitable. The question is, who gets to decide on when I die? You or me?
Whatever your concern about my agency as a future demented person, it is clearly not within the ambit of YOUR agency to make decisions about when I may or may not die if I have in the past made my considered decision clear.
If you say “But, I am able to think clearly, and in your state of dementia you are not able to understand these things. I think you are happy, so I will make the decision to override your previously expressed desires” - you are, in fact, supporting my assertion that when I believe I am thinking clearly, and I decide under what conditions my life should be ended, MY desires, not yours, should be honored.
If anyone is going to second-guess my future self, it should be me, not you. I promise you, if I get it “wrong”, that’s ok with both me and my future self. If my clear-headed early decision denies my later demented self one more tasty piece of pie or a laugh at the antics of a cat, that’s ok. If he could think clearly, and he could understand what was coming, he’d know I was doing it for his sake, and he’d be fine with it.
As an experiential note - have you ever gone to a nursing home and heard the old demented people strapped in their wheelchairs wailing? For hours? Or, in other facilities, seen them strapped in, slumped and silent, drugged so that they will not wail? That is the “agency” you are worried about violating. Can they ask for death at that point? Most of them can’t. And under your philosophical regime, they would be condemned to months or years of anguish before they die, because their past selves’ expressed wishes are not valid and they can no longer say “I don’t want to live like this any more”. That’s good for the nursing home profits, but I would assert that it’s bad for the old folks who are screaming and moaning or drugged into insensibility as they slowly dwindle towards their inevitable death.
I look forward to your response to these considerations.