I would like to point out that we're assuming that the Ansem Reports are 100% correct on the subjects they deal with. We also know that Heartless are able to use darkness and hearts to exist, so darkness has been shown to be capable of allowing a piece of a person to continue to exist. It is possible that such a thing could continue in this case, if darkness replaces the soul right off, maybe consumes it.
I personally like this theory. It explains things very well. If Master Xehanort is the one creating the Unbirths, that explains why they don't appear again (although it does leave the potential of them appearing in later games, I feel), and if they eventually change into Heartless (the darkness is strong enough to consume the soul, and then it starts on the body. The heart is not consumed by the darkness (hearts seem to still exist in darkness, even if they are used by them)) it explains where they went. Admitedly, if Xehanort is the source of all Heartless, then this theory has a few issues, but I feel that it could either be that the Unbirth didn't devolve into Heartless until way later, after Xehanort made them, or it could be that the Heartless existed before Xehanort started experimenting, and he just rediscovered and old enemy.
I say take the reports with a grain of salt. They are certainly the most accurate reports we have right now, in regards to the Heart, Body, and Soul, but things can change, I bet.
I personally like this theory. It explains things very well. If Master Xehanort is the one creating the Unbirths, that explains why they don't appear again (although it does leave the potential of them appearing in later games, I feel), and if they eventually change into Heartless (the darkness is strong enough to consume the soul, and then it starts on the body. The heart is not consumed by the darkness (hearts seem to still exist in darkness, even if they are used by them)) it explains where they went. Admitedly, if Xehanort is the source of all Heartless, then this theory has a few issues, but I feel that it could either be that the Unbirth didn't devolve into Heartless until way later, after Xehanort made them, or it could be that the Heartless existed before Xehanort started experimenting, and he just rediscovered and old enemy.
I say take the reports with a grain of salt. They are certainly the most accurate reports we have right now, in regards to the Heart, Body, and Soul, but things can change, I bet.