Alzheimer's creates a mental handicap that makes normal adult functioning impossible. You can literally ask the same question twice in two minutes and get two conflicting answers - with no memory of the previous answer, or even that the question was asked before.
In mid-stage Alzheimer's memory is so impaired that it's literally shorter than a spoken sentence. In late Alzheimer's even that disappears.
The solution in the UK is to hand over care decisions by Power of Attorney. If no relative is available for that, decisions are handed over to care and medical staff.
If assisted death is ever an option in the UK, it would have to be written into a living will while informed consent was still possible.
Obviously there are huge legal issues with that, especially if there's an inheritance involved.
That aside, people really do get to the point where they just want to die.
I've seen it twice now. In one case it was very clear indeed, and had been clear for at least a couple of years. In another it was less clear, but it was something my relative talked about openly.
Personally, having seen cancer and Alzheimer's close up, I'm really not convinced I'd want to wait around for a natural death from either.
Duress - could apply equally to euthanasia today too, couldn't it? If not, then take whatever magical anti-duress method we use now and apply it to this.