It's not about the technical complexity of the change. It's about the public blowback from such a change. The U.S. government compelling Apple to install some all-access backdoor would be a big story.

The U.S. government compelling Apple to make some small process change to this new system wouldn't be a big story by comparison.

Example: A year from now the U.S. government tells Apple they are no longer allowed to review the flagged CSAM imagery themselves but must rather report it directly to law enforcement.

A change most people wouldn't think twice about ("why does Apple need to be the ones reviewing this obscene illegal material anyway") yet would introduce a massive vulnerability into the surveillance process.

I don’t see what public blowback has to do with legal arguments, at least without also assuming some kind of legal realism.

I wasn't commenting on the law, rather the claim about the technical complexity of the change.