[Bullet Magnet]

Are you talking about brain dead vegetables or simply mentally handicapped? Because the latter get to live on account of being living human beings. This isn't an emotional argument, it is a bodily rights and human rights argument. Even if this were appropriate, where's the cut off point? how disabled do you have to be? Who decides which side of the line an individual is on? How would one protest the wrong ruling? It isn't just so absurdly unethical that your society plunges over the moral event horizon for permitting it, it's also so completely impractical that your meteoric plunge will be necessarily accelerated into the worst kind of oppressive hell.

If we're talking about actual vegetables then there's an argument to be made. But the guardians do have to permit it. A lot of the problems of organ shortage can be resolved with an opt-out presumed consent law. But that doesn't mean seizing organs from living human bodies independent of life support.

You seem to forget that human beings are emotional organisms and a lot of those emotions concern family. You're trying to take a sterile zero-sum approach and thus dismiss any arguments that have even a whiff of emotion. But in doing so you make a grave error: a society that does not consider the emotional health of its citizens is a broken one. Have you considered the possibility that being able to take away particular individuals from their families whenever they are deemed unfit to live may do more harm to society than good? What lengths might people go to keep such family members hidden? What mistakes might be made by the authorities? What social environment does this breed? What long-term effect might such unilateral action have on the surviving family members and thus the society in which they continue to live?

You can dismiss people's behaviour as irrational and pointless all you like, but if you therefore do not take it into account when you come to a decision about public policy then there will be unforeseen consequences. You need to work with the reality you actually have, not the one you think you should have.