I think where a terminally ill patient is in a position to ask for morphine or similar pain relief, they should be allowed it in amounts that will shorten their life, even to the extent of "kill them fast", if that's what they choose.
In a PVS/brain death case, it's just to make the onlookers feel better, the patient won't feel anything anyway (at least assuming the doctors are correct in their diagnosis).
The difficult case is the one where the patient can feel pain, but isn't in a position to express their wishes. I agree with you there is a danger of abuse, but think that maybe the case where the "form someone can sign" is a living will or similar signed by the person whose life is going to be ended, not by someone else, should override that.
Having said that, studies have shown many people dramatically underestimate how they cope with adversity, and will when healthy say they would rather die than be paralysed, only to find later that actually they are very glad to be alive, although paralysed. But for obvious reasons you can't find out how people who can't communicate and are never expected to be able to feel about it.