"Once you create demand for artificial content then someone is going to try and slip the real thing in, bringing us back to square one."
That is a slippery slope fallacy. Would you apply the same logic to legal pornography and say, "Well the actors are just too young, someone is going to try and slip in underage actors?" Or to non-visual descriptions e.g. Nobakov? At what point do you think the line should not be expanded?
"Artificial depictions of child porn are already illegal in many jurisdictions for that reason."
I think the reason is a lot less rational. In the 80s and 90s we had a widespread moral panic about child molesters that results in thousands of innocent people being thrown in jail. We still see remnants of that panic. I think we have shifted from a rational motivation for protecting children to a moral motivation to jail pedophiles. People do not want to have pedophiles in their communities and if a pedophile is able to avoid breaking the law people demand a broadening of the law. The idea that a pedophile could avoid prosecution by satisfying himself with cartoons instead of recordings of child abuse led to the law being broadened to ensure that the pedophile is punished.