I'm seeing Russ' point. Lise, sadly by the time a person has reached this point, the loved one you are trying to hang on to has already left you, in mind, soul and spirit.
I also don't think anyone is refusing treatment when they know they can do something. On the contrary, they are refusing treatment because it is absolutely certain, that they can't do anything. I think it may be harder to face our loved ones being alive..... but 'not there' AND with no hope of their return, than it is to face their passing.
As for a doctor's responsibility to prolong life .....at a certain point, that is no longer their job. I think that order comes down from a higher power.....and if doctors had to ability how and when to accept this, they might be able to make more rational decisions. Who would benefit from prolonging a life that remains only because of the valiant attempts to cheat death (the ventilator) ?
Having said all that.....I still wouldn't want to be the one faced with that decision.