If you do not have one in place the doctors will do what they want.
I rather doubt I was lucky. Beside my two experiences, I have family members with similar ones. Yes, there are some doctors who will push their authority, but it's rare despite the sensational cases that hit the news. To me, your post made doctors sound much like Obama did when he talked about unnecessary tonsillectomies. Doctors, being human and all, have their own strengths and weaknesses, but I don't believe as a group they all want to play God.
And, yes, a living will and care directives are a good idea, but if someone doesn't have on it doesn't mean they doctors will take carte blanche.
Not all will -— but the best thing ( and that is ALL I was saying) is to be proactive in your own health care.