Much of the discussion is about how trivial it would be for Apple to start scanning any photos on the phone at a later date.
Right now they are able to bill this as doing what they currently do server side, but client side. Later, they can say they are simply applying the same "protections" to all photos instead of merely the ones being uploaded to iCloud.
They can do it already. System is full black box, and all we have is their word. So, saying that adding something might enable something else, is not strong argument.
By that logic we may as well never question anything any of these companies - or even governments really - do because they might just find a way to do it secretly and maybe nobody would ever figure it out.
Right now they are able to bill this as doing what they currently do server side, but client side. Later, they can say they are simply applying the same "protections" to all photos instead of merely the ones being uploaded to iCloud.