I suppose one could say that Mrs. Schiavo died a natural death, since if not for intervention by artificial medical technology, she would have expired back in 1990. But that's irrelevant. The important point is whether she died a death she'd have preferred, given her condition.
And in that case, I think we might possibly have let her down...not allowing a more humane active passing, rather than having to resort to "natural" death by removal of hydration. That's a distortion of prinicples, implying that disAbled people are somehow kept alive "artificially" if they require assistance. Again, we must focus on the rights, not on the "natural/artificial" aspects.