If fully-encrypted private messages would provide a threat to children, because it would enable people to interact with them on your platform without surveillance, then children should not be allowed on your platform and/or have access to private messaging features.

This isn't an argument against encryption, it's an argument against unsupervised children on your service.

If you don't want that because it would hurt profits... well. That's something else entirely.

@kraln no no, it's okay. Kicking out children is the easiest way to do this. Just add a checkbox somewhere that you have to be over 18 and go for it. Or just update the terms and conditions. Then you don't even need to change anything in the frontend. (/s)

And then you can continue to role out shit for the adults.

@fuchsi the checkbox asserting that the user is of legal age in their jurisdiction unfortunately leads to the current "send all of your personally-identifiable information to a third party so we can confirm your age but also do a bunch of evil" which is going around at the moment.
@kraln fits into the business models of most "we sell ads" companies