It's the error of misguided autonomy. There has to be autonomy.
I can say at I want to buy a desert island in 10 years and wind up alone in a house, but, in the end, I have a large family and friends. Will I really want to live out my life alone on a desert island?
Perhaps my example's somewhat ridiculous, but we don't know what the future holds for us. Can we really anticipate dementia and the kinds of symptoms and psychological behaviours associated with it? People who think they can should provide some compelling evidence to confirm it. We don't know the future when a diagnosis is made. We don't know when the dementia will be advanced.