That’s like building a tank and saying it’ll only be used for deliveries. Once the infrastructure is in place on every device to detect arbitrary images/messages on the device the switch will be flipped.
iOS is not open source, so, Apple could just deliver the spy payload with another update and probably nobody would ever know if that was the intention.
The slippery slope argument simply don't make much sense here (as usual, btw)
You can keep that secret until someone goes to court or has police raid their house over photos that were supposedly end to end encrypted.
Like whatsapp saying their chat is encrypted, is it really? Well, Facebook is trying really hard to losen this up, why don't they just release an update sending them the keys?