It shouldn't hinge on whether one can speak. If the wishes are clear, they're clear. If not, the mistake should be made on allowing another full review of the facts. Hopefully there are not three people in my life or history that would sign affidavits saying I wanted something I did not want (oh, and signing those affidavits years after the fact). There are plenty who cannot speak, so,,,they die?
The issue, certainly, is whether Terri ever expressed her wishes prior to her injury, but that has nothing to do with Christopher Reeve's type of injury and there's no analogy.
You said, "If the wishes are clear, they're clear," and that's exactly how the cases differ. His were, hers weren't, and that's how the courts got involved.