We can carry every single element back to its inception and make
identical arguments. That's the problem with slippery slopes.
Apple - creates messaging platform.
Slippery slope - governments can force Apple to send them all messages.
Apple - creates encrypted messaging platform.
Slippery slope - governments can force Apple to send them the keys and all messages
Apple - Adds camera to device (GPS, microphone, accelerometer, step detection, etc)
Slippery slope - governments can force them to record whenever the government demands and send a live stream to the government. Or send locations, or walking patterns, or conversations.
Apple - makes operating system
Slippery slope - Basically anything. There are so many ways I could go with this. Government can force them to make a messaging and photo system, add cameras to their devices, entice users to take and accumulate pictures, and do image hashing and report suspect photos.
That's the problem with slippery slope arguments. They become meaningless rhetoric.
Image perceptual hashing is literally a single person's work for two hours. If you really think the barrier between an oppressive government stomping on groups or not is whether Apple did a (poorly communicated) Child Safety PR move and implemented this trivial mechanism...the world is a lot more perilous and scary than you think.