The right to life is the fundamental right in this case.
Her wishes are immaterial. The right to life is inalienable - it cannot be legitimately surrendered (that's what inalienable means). Moreover, her wishes are not established with any degree of certainty, and most surely not with sufficient certainty deprive her of the right to life.
In any other case where the government deprives a citizen of the right to life, the standard is "beyond a reasonable doubt". And indeed, Terri Schiavo deserves no less than a jury trial for her capital crime of being injured and unable to speak for herself.
See, it didn't take us long to get there.
Your stated view is that, if Terri were able to communicate and clearly stated "Disconnect the tube," it wouldn't matter - the tube would stay in. That may be your view, but it isn't Florida law. Nor is is Federal law.
The Soviet Union is kaput, but there's still North Korea!