Unplugging someone who appears brain dead or completely comatose from a respirator is quite different from withholding food and water from someone where there is disagreement just how conscious the patient is.
If a person were to become quadraplegic and could no longer feed him/herself, would you say "Set the food in front of them, if they can feed themselves, they live. If not they die." I doubt you would.
Simply being unable to feed oneself seems a rather callous measurement as to whether they should live or die.