I can understand fully what you're getting at & if everything was black and white it would be easy enough to possibly pull off. 

.......

but, it's sort of a slippery slope as well.  Who declares an individual to be demented and how it is defined exactly are 2 really gray and fuzzy areas that could easily be abused by folks with less than good intentions.

Not to mention some folks just wouldn't stand for it when they were in the beginning stages of alzhiemers or the like.  "Invasion of privacy" screams out. 

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if everyone had a good heart and the best of intentions, it would be adopted by all at the drop of a hat, but that's not the case.