I agree there is a slippery slope concern, and Apple has themselves made related arguments such as the FBI case and not wanting to create a software update to decrypt the contents of a phone. However that is contrasted with a very real and much more practical concern of malicious parties getting access to your cloud stored photos.

It would help to note Apple also today announced "Advanced Data Protection" in iCloud which closes the hole where iCloud Backups, iCloud Photos and various other bits of data were technically decryptable by Apple. They've closed that (but it's opt-in, to balance the average user losing all their photos against other users desire to be secure even if it means losing their data). Details: https://support.apple.com/en-us/HT202303#advanced

However even without the "Advanced Data Protection" what I said about not having a workflow for any Apple server to "normally" request both the keys and photo data is also still good security.