Your question tries to over-simplify an extremely complex issue.
The answer would depend on a setailed analysis of a wide range of associated circumstances involved in the specific case.
Let me reverse the question: Do you think it is reasonable to artificially sustain such a person for many years when it's possible or even probable this would be against their wishes. (To me, and for me, such a fate is much worse than death.)
In either case, I think that the legally designated decision maker can and should apply criteria to the set of circumstances that they would use if making such a decision about theirself - i.e., what their own decision would be about theirself if they were in such a situation.