Do you think it's more as Rivka said above, they don't have the right to get someone else involved that way? Either to insist that they can have someone who has consented to help them die later do so, or increase someone's risk of nonconsensually killing them in a car accident? I find it easy to see the argument that Person X's dementia doesn't give them the right to involve a total stranger in accidentally injuring them. I'm not sure I agree with it but I can see it.