Killing adults is less reversible in the sense that if you kill comedian carlos mencia, you can't get a new carlos mencia if you change your mind. In contrast, babies are basically fungible.
In contrast, babies are basically fungible.
This doesn't follow. Just because you don't have data about what counterfactually the baby would have turned into doesn't mean that the babies are fungible. We don't in general keep the genetic code of aborted babies or calculate how that would interact with their environment. Just because we can't easily predict what the distinctions would be doesn't mean that the babies are all identical.
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.