I am sure that you don't believe that we have the best legal system that we possibly could have, nor that we can't do anything to make it better.
Dred Scott was decided according to established procedure, but that didn't make it right. Good men didn't throw up their hands and say that there was nothing they could do about it, either.
I think we'll have to accept that the law will never be perfect, that there will always be edge cases which find cracks in the system. That being basically inevitable, it would, it seems to me, behoove us to ask whether some particular attempt to perfect the system further does more harm than good. The very same probate and guardianship procedures that may have failed Terri Schiavo daily serve thousands upon thousands of others in a satisfactory manner. I submit that the price of even significantly reducing the chances of this ever happening again are higher than most will be willing to bear, for one simple reason - she's an absolute edge case. She is not reflective of all, most, or even many families in this position, and before we start rebuilding the law based on her particulars, perhaps we ought start asking ourselves what the impact will be on all those people the law is serving reasonably well.
Lawyers, as I seem to recall, have a pithy little saying about building laws based on the edge cases - "bad cases make bad laws". You can change the law to insure that no case like Terri Schiavo will never, ever happen again, but those changes will not be free, and someone will bear the costs. Are those costs really worth the benefits they bring? I don't know. I tend to doubt it, but I've noticed a certain allergy to even asking that question around here lately.