Now my instinct (keeping in mind I don't have children or a wife or any such external responsibilities) would be to off myself as immediately as possible. I'm comfortable being my own judge rather than someone else's
But see, that's just it, man - this isn't about judging anyone. It's about making the most economical, well-reasoned, and, most importantly,
humane decision possible when presented with an unambiguously shitty set of alternative options. In other words, it is merely a matter of optimization, viz., the maximization of the patient's well being and the minimization of healthcare cost. As I see it, if a patient is incapable of scoring higher than an 8 on an MMSE for months on end, their mental state is not necessarily comparable to that of someone under the influence of a powerful hallucinogen, and at any rate is simply not capable of meeting the cognitive demands of a full, healthy human life. In these instances (i.e., comas, rapidly advancing neurodegenerative diseases), there aren't very many alternative courses of action available for anyone involved: In the case of the Alzheimer's patient, you can either 'execute' them or let them experience the protracted torment of literally
losing their minds - friends, family, life experiences, even how to perform basic, everyday tasks. Please note, however, that I do not, by any means, wish to appoint myself the sole arbiter in such regrettable cases as these. The decision should, of course, be left to a spouse and/or the closest blood relatives of the afflicted (or a third party stipulated in a will).