Oh, I think I misunderstood you. I thought you meant instead of "sending images that collides with perceptual hashes of known CASM", why not "send actual CSAM in 'Facebook Messenger or Gmail or Dropbox', and since those services also use some other detection algorithm, it will also incriminate the receiver."
Those services will take your account through the same, if not more invasive, process if you are found with a hash match like the ones being proposed in these comments. Unlike Apple, they’ve built interfaces that surface all your account activity to reviewers.
In some ways, you can start to see the value in Apple’s system which lets the device user inspect what is stored in the associated data for later review.
