Per the threat model most of the people on here seem to be buying into, Apple can do whatever they want in an iOS update, including sending iCloud Keychain secret keys to a server they control.
The threat model people are buying into here is that Apple can’t do what they want today, but if they deploy this CSAM detector, only then will they be able to do whatever they want.
The threat model is that Apple has been capable of doing whatever they want, and this CSAM detector has demonstrated that Apple will do whatever they want.
Then by that logic, what they want is build a complex and highly narrow mechanism for checking only the photos that are uploaded to iCloud Photo Library for CSAM, and absolutely nothing more.
Today. They didn't want to do that yesterday. They will certainly want to do something else tomorrow.
Apple has a history of modifying devices that people bought in ways they didn't want and could not change (like putting a non-removable News app on their computers), but the CSAM episode shows Apple is willing to do a lot more, and more importantly, explained this to users who didn't care about the past abuses.