Moving it on device will show 0 improvement to the original goal, while opening a door that quite frankly I never expected Apple to be the one to open (I would have bet on Microsoft).

The CSAM scan is only for photos that are to be uploaded to iCloud Photos. Turning off iCloud Photos will disable this.

My point is that to my knowledge, this is the first time that an on device "content check" is being done (even if it's just for photos that will end up in iCloud). This is the precedent (the on device check) that makes me and some others uneasy, as pointed out in the linked letter. The fact that it applies only to photos going to the cloud is an implementation detail of the demonstrated technology.

Legislators around the world now have a precedent and may (legitimately) want it extended to comply with their existing or upcoming laws. This is not a particularly far fetched scenario if you consider that Apple has already accommodated how they run their services locally (as they should, they have to comply with local laws around the world in order to be able to operate).

That's the crux of the issue most of the people quoted in the letter have, one can argue it's just a slippery slope argument, I personally think that one can be legitimately concerned of the precedent being set.

Keeping doing it on server, in my opinion, was a much better option for users (with the same compliance to local laws and effectiveness to the stated goal as far as we know, there's no improvement on that front, or none that couldn't have been brought to the existing server check), and ultimately also a safer option in the long run for Apple.

They've opened themselves, for little reason, to a large amount of trouble on an international scale and at this point rolling it back (to server checks) might not make a difference anyway.