That's not the only criteria. Even a "scientifically" hopeless case can still live a life with some meaning: based on who they were, who still loves them, whether they can respond in even a small way, or whether they have some awareness of their surroundings. We just don't pass laws that officially take "miracles" into account. The situation you describe is a medical decision. And it seems her doctors are convinced - her parents and their alternative therapists disagree. What's a judge to do?
The real question is what would/does Terri want? And if she had gotten a living will we would all know. Absent one, the decision falls to the husband (right or wrong).
That's why everyone should:
a) Have a living will and/or:
b) Marry better.
I think it's a moral decision, not a medical one. There are no medical tests for whether someone is loved in whatever state they may be in. There aren't very good medical tests for whether someone may be conscious; as shown in other threads people may be conscious but unable to express themselves.
The main problem is that medicine, like all science, is amoral.