"It won't catch all of them, of course. My point is that it is invalid to say that it won't catch anyone."

Sure, but wouldn't they quickly learn once people are getting caught?

No, they wouldn't. People were getting caught before encrypted apps. People are still getting caught on unencrypted apps today, even if it's easy to install an encrypted app.

And predators who get in contact with kids have to do it over social media that the kids use. Those ones would be affected by ChatControl.