If you think Apple is on a slippery slope and will just expand this feature without any consideration then why have an issue now ?
Apple already has your unencrypted photos. They could scan it server-side. Or they scan it on your device and simply not tell you. And they can push OS updates without you knowing to enable all of this.
Apple could even push CSAM to your phone and frame you if they wanted to. They control ALL of the keys to your device whilst you are using iCloud and allowing software updates.