I've heard advisors I trust caution that "living wills" are not necessarily a good thing.
One problem is that a brain-injured person's true prognosis is often very difficult to determine. Some doctor's and "expert's" opinions are biased, negligent, or just plain wrong.
So if you ask most people if they'd like to have a "living will" wherein they ask to be refused all treatment if they're in a "vegetative state," most people will probably say 'yes.'
Yet those who say 'yes' to termination of treatment are often ill-informed. They may not realize that many, many seemingly hopeless cases of brain-injury and coma have turned out for the better, with the injured person recovering well and living many happy years more.
I know personally of a case where doctors pronounced a patient's case hopeless; yet that patient is now a healthy, fully-functioning person 30 years later! What would've happened if he'd had a "living will"? I shudder to think.