People on average increase in societal value from conception to childhood, and then it gets more complicated from there depending on how they turn out. And yes, typically their value declines as they become elderly.
But, as in your example with your adopted friend, even a baby that starts out unwanted, if society invests a bit in its welfare, will soon become part of the social fabric and so on and thereby become valued.
Certainly there are some people who literally nobody likes, but even then, there's still reason B.
As it happens, my best friend was adopted as well. But I hardly think the limiting factor in the number or quality of my friends is society's production of babies.
A few years ago, I wrote a little dialogue I imagined between 2 materialists, one of whom was for and one against abortion, centering on the personal identity question. I recently cleaned it up and added a number of references for the biological claims.
You can read it at An Abortion Dialogue.
Early feedback from #lesswrong is that it's a 'nicely enjoyable read' and 'quite good'. I hope everyone likes it, even if it doesn't exactly break new philosophical ground.