Let's think a little more carefully for a minute. The issue is not whether Terri was 'brain dead'. We know (and all the experts agreed) that her brain was basically gone and replaced by spinal fluid, but that her brain stem was still functioning (otherwise she wouldn't have been able to breathe).
The issue was what does this mean for her quality of life? The respondents' experts argued that brain cells can sometimes be trained to do things that they don't do in normal humans. Their argument was that some of the brain cells in the brain stem might be retrained, by means of certain experimental therapies, to handle very elemental speech and/or motor functions.
By the way, on your 'unanimity' point, even the two respondents' experts did not agree. Each had his own 'therapy' which he thought might work.
In short, the issue was not 'brain death', but whether or not (whatever the cause of her condition) Terri would or could 'recover.' Now "recovery" is a continuum not a point. Moreover, it is prospective and contingent so there can never be unanimity on anyone's part. Here is the Court of Appeal's summary of that evidence:
The evidence is overwhelming that Theresa is in a permanent or persistent vegetative state. It is important to understand that a persistent vegetative state is not simply a coma. [FN1] She is not asleep. She has cycles of apparent wakefulness and apparent sleep without any cognition or awareness. As she breathes, she often makes moaning sounds. Theresa has severe contractures of her hands, elbows, knees, and feet. {Par.] Over the span of this last decade, Theresa's brain has deteriorated because of the lack of oxygen it suffered at the time of the heart attack. By mid 1996, the CAT scans of her brain showed a severely abnormal structure. At this point, much of her cerebral cortex is simply gone and has been replaced by cerebral spinal fluid. [Par.] Medicine cannot cure this condition. Unless an act of God, a true miracle, were to recreate her brain, Theresa will always remain in an unconscious, reflexive state, totally dependent upon others to feed her and care for her most private needs. She could remain in this state for many years."
That was the evidence; everything else is speculation.
But what does this mean? Did Terri want to live like that? That was the key question.
And the court determined that she did not. We have absolutely no reason -- grounded in logic -- to think the court was wrong.