I think you overestimate the technical knowledge and skills of many people. For every crafty career criminal there's a dumb idiot doing dumb stuff on account of being a dumb idiot. And even crafty career criminals can and do make mistakes.

Of course it won't catch 100% of it, but very few laws or measure do that (and those that do almost always come with very high amounts of false positives). Also links are not hard to scan for either, and you can't "just" upload CSAM to anywhere either and not expect troubles sooner or later (unless you know what you're doing, in which case we're back to the previous paragraph).

I think it's very hard to deny that of course it will catch a non-zero number of people, although it's hard to determine exactly how many beforehand. That doesn't mean it's a good law, it just means that all these type of "it won't even catch anyone!" argument are just bad arguments.