My opinion about having a living will is based on mainly on my experience. Although I work in the legal field I am not a lawyer. You should take care not to mistake anything I say as legal advice. With that out of the way, I can express freely that a living will not do all that you may think it can.
Because humans fear death, we try to prevent our own demise even when it is inevitable. With life saving equipment and mechanical life support, it is possible to thwart the natural process of death and suspend life for years. Make sure your end of life decisions regarding life support and resuscitation are honored. Get a living will NOW and ask someone to facilitate who is able to carry out your desires even under emotional pressure.