The underlying idea here seems to be that if there's some chance of full recovery, one should not wish to be let go.

Is it better for 100 families to live for years with a vegetative loved one with the most realistic hope being that a few to emerge profoundly affected and never their full selves again, or is it better for those hundred families to get to grieve?

The pain of a loved one's continued quasi-existence, plus the difficulty of their life if they ever are to recover, make it so that the compassionate personal choice is to say "once the best estimated probability of my recovering robustly is clearly below P%, let me go". The value of P is a decision to be made carefully, and with deep consideration for ourselves, our loved ones, and for all of humanity.