They have plausible deniability, but the fact of the matter is: this also erases evidence of past crimes from public records. If bad things already happened then we should keep the evidence that they happened.

The root problem of CSAM is child trafficking and abuse in physical space. But for whatever reason enforcement efforts seem to be more focused on censoring and deleting the images rather than on curbing the actual act of child trafficking and rape. It's almost as if viewing (or this case, merely archiving) CSAM is considered a worse crime than the physical act of trafficking and sexually abusing children, which is apparently okay nowadays if you're rich or powerful enough.