I am totally in favour of this- and very lucky that I live in a country were such wills are properly recognised and legal, and go much further, allowing you to make a request to die if you feel the pain is too much. But even here, you still need to have all your mental faculties at the time of the decision, and not only at the time of making the will. With Alzheimers and senility, it still means people have to 'exit' much earlier than wished, just to make sure you are still considered compus mentis.
Suggestion for anyone about to embark on making a will or engaging in the planned chat on 18 April. Have you thought about a living will to specify exactly how you want to be treated in the event of terminal incapacity? I have recently completed the legal process myself and a copy is then lodged on your medical file with your GP. Please note this can only be done when you are of sound mind, so if you are getting on a bit don't leave it to late. Your lawyer has all the forms etc. and. my doctor was very supportive. I have seen the alternative a few times and it is not pleasant.