Not to say that we need to scan messages to enforce nudes not to be sent, but I don’t think you can say “just enforce existing laws” and be done with it, it’s not that simple.
The lack of proper enforcement breeds distrust. The government already has access to WAY more data than it needs and... nothing is happening? People are still getting unsolicited dick pics and nobody seems to care?
The only reasonable conclusion is that the government wants the data because it wants it. The crime angle is just one that is easy for people to swallow. See also: think of the children!
You don't.