You are right about one thing. Having been in the position of having to make a couple of these decisions you will never convince me that you have the right to decide this for anyone except your own immediate family. Many millions of us have been there and done that. The odds are that if you haven't had to live that part of life yet then it is only a matter of time. I hope people let you alone when you turn comes.
I'm sorry you have had to go through such trauma. I imagined you had. I have also personally experienced this and have witnessed 3 comas in friends whose families never gave up. Two recovered; one made continual improvements prior to succumbing to illness.
Regardless of the "just in case" reasoning, pain medications are sometimes a means for more expedient degeneration.
No one, let me emphasize NO ONE should ever have the right to take away the life of a mother's child, NO MATTER the child's age. You can't get more "immediate" than mother/child.
Again, I am sorry for what you went through. My intention is not to cause you to feel defensive. It is rather to defend the god-given right of a mother who wants to care for her needy, once-again-dependent child.