The practical argument is that whatever dark-market ended up trading models which produce CP material would likely end up being spiked with actual CP input data

The practical argument is that whatever dark-market ended up trading models which produce CP material would likely end up being spiked with actual CP input data to try and improve "realism". At which point the moral dimension clarifies: creating an incentive to abuse children to create better models for simulating child abuse is the exact same reason posession of CP is illegal to start with. And since it won't be possible to prove any particular model without perfect providence wasn't produced this way, their use will just be illegal. This is, realistically, also the problem with just pornography generating models in general, and all of these models in total: if there's illegal data in the input set, what does that mean? I suspect the next hurdle with them is going to be license compliance-enabled training - models which can prove any given output has at most a certain type of license and providence on input data.