You never know bout Coma patients, there's been a case of a woman waking up after years of Coma.
The problem is that medicine cannot operate on a "you might never know" basis. Hospitals around the world in this day and age are purposefully managed to be
underbedded, in an attempt to cut cost and reduce bedding spans. That has the consequence that your 80-yo zero-score grannie is competing for an ICU bed with the young girl with the bicycle incident they just picked up; and at this point the question becomes just a bit more brutal.
Do you turn off the machines on the grannie and let her die? Or do you put the girl in a normal bed and let her croak? Decisions, decisions.
I personally think that if a person is deemed clinically dead with no realistic chance of survival, they ought to be switched off stat. Relatives ought to have no say in it, as it is a well-known psychological fact that they will often refuse to have a loved one switched off, because they hate to let go. That's understandable, and to some point human, I guess, but it ought to play no role in the matter at hand.