The real problem is that we don't have a set of ruleslegal or otherwiseupon which we can rely in determining what someone's wishes would have been in an unforeseen set of circumstances when that someone was silent in advance. Instead, we're trying to work it through the legal system, both in the courts and elsewhere. For what it's worth, that's probably a better choice than some eclesiastical or peer-pressure-based system; that doesn't make it good enough. Projecting one's own abstract desires onto the concrete situation is worthless. It's a problem that those of us with command experience understand all too well: Say all that you wish about "good management practices," "good tactics," or whatever, the commander on the spot is the only one who can make most decisions… and those who blithely declare, with 20-20 hindsight, that they would have done differently are being either intellectually dishonest or just plain stupid.
Of course, I live in a state that explicitly disallows living will provisions that call for withdrawal of nutrition and hydration, instead committing that decision to (at best) someone with a power of attorney. But then, nobody has ever accused Illinois of being on the cutting edge of legal doctrine.