Zahavasdad,
If PBA were in need of a heart transplant, by your thought process he would be entirely within the right to take insist that a doctor harvest yours. Althoigh you are alive, your brain function seems to be less than his and as such he would be able to kill you for it.
Which is clearly not the halacha. Even if he wanted to be meikil.
The classic case of Yehoreg va’al yavor is exactly in these cases. Can you kill someone to save your life. Can you kill someone by harvesting their organs (even if it is not a vital organ, it is clear that they are actually finalizing the death process by harvesting the organs) to use for yourself.
It is very clear that the amount of life a person has left is not a determinent factor in what is or is not retzicha. When they take organs from a person who is not halachically dead, they are killing him. It makes no difference is one person is a mother of eight and the other is unfortunaltely a vegetable.
Where the organ comes from makes a difference, do not try to use that argument as well as what to do once the organ has been harvested.