How do you know that these persons are in pain? Just because they dont function on a normal human level does not mean they are in pain, nor does it mean they waive the right to have a life.
Would you like to live like that? No?
Imagine, never being able to truly understand and comprehend, never being able to be independent.......truly, that is a life of suffering.
If we could, however, detect disabilities in the womb and thus abort before birth, it'd be ideal as the "baby" is nonexistent.