Your opinion on life, and any judgment you may be willing to pass over another human being's life without their direct consent does not matter.
And whose opinion matters then? Your opinion that the default is to keep everyone alive?
The trouble is, that this girl herself most likely has no way of communicating with outside world. It's even possible that she doesn't have anything to communicate, that her existence consists solely of pain. She has no conscious body movement, her sight and hearing are impared. Thus I doubt that she has ever learned to, for example, understand speech and develop the skill of abstract thinking.
Obviously, we can't just ask her whether or not she has something to live for. Personally I doubt it. Her parents were of different opinion and wanted to keep her alive, and so she was kept alive.
But like you say, neither their or mine or anyone else's opinion is the only right one. The right opinion would be what
she wants, but as shown, it's impossible to know what she would want, if she has somehow managed to develope any kind of sentient awareness and abstract thought.
So it is up to other people to decide what's best for her. Normally it's up to parents to decide that, but in cases where parents' decisions cause suffering to child/ren, it is normal to intervene and prevent further suffering. Granted, this is a difficult issue, and I really really hope that I'm never going to be in a position to make decision like this, but I think that I would've agreed with the doctors in their analysis that it would be rather pointless to maintain the bodily functions of the baby, since there never was much promise of tolerable human life for her.
It is, of course, easy to see in retrospect that the decision to keep the girl alive has done no good but instead just prolonged her suffering and contributed to parents' separation. And that is the greatest problem of consequential ethics in general. Deontological ethics gives much better basis to make decisions like this. Would you appreciate being kept alive and never being able to live? To be unable to do anything but suffer until death comes?
I for one wouldn't really value it if I had been kept alive just because my mum and dad couldn't face the truth that I would never be able to live despite being alive. Not that I had ever known about it, as I suspect the poor girl is unaware of anything but pain.
Darkness - imprisoning me
All that I see
Absolute horror
I cannot live
I cannot die
Trapped in myself
Body my holding cell...