When someone testifies in court, the "finder of fact" - whether the judge or a jury - determines the credibility of that witness. If MS testified that Terri said she wouldn't want a feeding tube, there's no evidence to contradict the statement, and the finder of fact found his testimony credible, then it is accepted as fact and may be factored into the verdict or ruling. If we can reject court decrees just because we don't happen to like the outcome, regardless of the evidence, there is anarchy. I reject anarchy, and I'm pretty sure our founding fathers would, too.
Thomas Sowell the other day called the law a blunt instrument at best, but sometimes it's the only instrument available. And it's approximation of justice is often the best you will be able to get, even if you don't like the results.