If death is ever preferable to life you can always kill yourself. Having the option to live or die is strictly better than having no option.
My point regarding having the brain of the 90 year old is not that that would be good, but that it would be better than having the brain and body of a 90 year old. I don't want to be frail and prone to injury if I could be healthy and strong.
You could kill yourself before it advanced that far or set a living will. You'd possibly need to move to a country that would be willing to euthanize you - but you could do it.
My point regarding having the brain of the 90 year old is not that that would be good, but that it would be better than having the brain and body of a 90 year old. I don't want to be frail and prone to injury if I could be healthy and strong.