I mostly agree with you, but a counterpoint:
Downloading and possession of CSAM seems to be a common first step in a person initiating communication with a minor with the intent to meet up and abuse them. I've read many articles over the years about men getting arrested for trying to meet up with minors, and one thing that shows up pretty often in these articles is the perpetrator admitting to downloading CSAM for years until deciding the fantasy wasn't enough anymore. They become comfortable enough with it that it loses its taboo and they feel emboldened to take the next step.
CSAM possession is illegal because possession directly supports creation, and creation is inherently abusive and exploitative of real people, and generating it from a model that was trained on non-abusive content probably isn't exploitative, but there's a legitimate question as to whether we as a society decide it's associated closely enough with real world harms that it should be banned.
Not an easy question for sure, and it's one that deserves to be answered using empirical data, but I imagine the vast majority of Americans would flatly reject a nuanced view on this issue.
True, it wouldn't be ethical to conduct an experiment, but we can (and probably do) collect lots of observational data that can provide meaningful insight. People are arrested at all stages of CSAM related offenses from just possession, distribution, solicitation, and active abuse.
While observation and correlations are inherently weaker than experimental data, they can at least provide some insight. For example, "what percentage of those only in possession of artificially generated CSAM for at least one year go on to solicit minors" vs. "real" CSAM.
If it seems that artificial CSAM is associated with a lower rate of solicitation, or if it ends up decreasing overall demand for "real" CSAM, then keeping it legal might provide a real net benefit to society and its most vulnerable even if it's pretty icky.
That said, I have a nagging suspicion that the thing many abusers like most about CSAM is that it's a real person and that the artificial stuff won't do it for them at all. There's also the risk that artificial CSAM reduces the taboo of CSAM and can be an on-ramp to more harmful materials for those with pedophilic tendencies that they otherwise are able to suppress. But it's still way too early to know either way.