Again, the film was not about abortion....

A lot of people think that, but, the film is only showing that the main character felt bad about having an abortion.
I think a lot of people didn't understood that the world created by the lady writer was her own version: So it doesn't mean that all the abandoned things (including ideas or aborted kids) of all the people or writers in the world ended there, the world was created by her and for her in the story she wrote, that was his own version, a place like that.
It was not something universal.
When the guy inside the world told her that there are more of you, he meant more versions of her (as we saw in the last scene of the film) he was not talking about all the writers of the real world.
A film can have a character that felt bad about an abortion, but it doesn't mean that the creators of the film are sending a message, it was just an aspect of that character.