So you would impose medical treatment on those who do not want it? Should cancer patients be required to undergo chemotherapy? Should heart patients be required to undergo surgery? What about the risks and side-effects of treatment? At what point does "duration of life" supersede "quality of life" as a value? And why should the government or doctors be making those decisions for people?
So restate -- If a person is obviously in such a mental state as to not be able to make a proper contract -- then the refusal should not so easily be accepted -- efforts should be made to suggest -- strongly -- that the patient should accept what will be needed to live. But is the refusal continues even in the face of that cajoling -- it's a refusal. If the person is greviously impaired -- drugged, unconscious or nearly so, seriously dazed and refuses then yes treatment could be started under the assumption that the refusal is ignorable.
But a advance DNR order? That's a vile document unless the person is clearly in their last few days of life -- and even then why not hydrate?