I'm personally highly skeptical of the "offering them an outlet" argument. I'd be less suspicious of the idea if its proponents also suggested limiting it to controlled settings, e.g. during meetings with a professional psychiatrist.
But I'm sorry, I just don't believe anyone holed up in their room with a bunch of fake CSAM is "just using it as an outlet" or "protecting real kids from harm." I mean, it almost sounds like a threat: "If you don't let me look at these pictures of fake kids, I'll hurt real kids." If that's the case then they should be seeing a psychiatrist, at minimum.
Violent movies that appeal to teens reduce vandalism and the like--they're in the theater rather than out causing trouble. (And it's not displaced, rates don't spike later, they just return to normal.)
But I'm sorry, I just don't believe anyone holed up in their room with a bunch of fake CSAM is "just using it as an outlet" or "protecting real kids from harm." I mean, it almost sounds like a threat: "If you don't let me look at these pictures of fake kids, I'll hurt real kids." If that's the case then they should be seeing a psychiatrist, at minimum.