I think this is a highly individual thing. I personally think that this is mainly a question about what life really is. If it were mere existence, then letting people hang on automats for a few years would be an act of grace and it would be worth the price. If it is (as I am convinced) all about experience, growth, learning and joy, then it is a crime to keep the body alive when the mind has already gone.
For myself I can say, I wouldn't want to be kept alive artificially over a longer period of time, if my mind doesn't recover from what it is, then I'd want to go. And no one, NOBODY should ever have the right to decide over my life, exept myself and God. Even suicide is my decision, and in my eyes, it is only so highly chastised as a means of suppression (how could you ever enslave someone if he could just kill himself?).
Most people with cronical, life threatening illnesses would be dead since a long time anyway, so if they cannot make it artificially, how could that be suicide? So, to answer your question precisely, for me it is certainly letting nature take its long overdue course.