will never be able to care for herself, speak or even feed herself. Everyone who is conceived has a right to be born and live their natural lifespan.
It's quite a stretch to speak of a "natural lifespan" when discussing a 20+ year old who has never been able to feed herself. If people freely choose to unnaturally extend the lifespans of severely ill or disabled relatives or strangers, at their own expense, and as long as it's not in contravention of the expressed wishes of the person being sustained, that's fine. But this unfortunate young lady's "natural lifespan" ended a couple of decades ago.
Why do you say her "natural lifespan" (your quotation marks) ended a couple decades ago because she needs care almost like a small baby? What about old people who can't care for themselves? Or people who are sick and can't care for themselves? Sometimes people are sick for a year or two, and then recover. Is your standard of whether someone should be allowed to live that they are independent and need no help?
It sounds as though you are saying the natural lifespan stops when a person needs help. Do you admire the practices of various aboriginal peoples who leave their old people to die alone, once they become feeble?