At least for the one I have, you can specify that you want to be kept alive.
that's the one I would want. But what can you do if your legal guardian tells the doctor to manipulate the facts so they can pull the plug? If your money has run out and your insurance company won't pay? If your relatives want what little money you have left?
That didn't used to be necessary. It was understood that patients were to be treated humanely. Very few people wanted to forgo procedures that the majority of us consider basic care (like food and water.) They were the ones who had to put those wishes in writing, because nobody would believe those were actually their wishes without irrefutable proof. What happened?