It's an interesting idea. Assuming it's feasible, I wonder whether people would actually want to use a messaging service that's private, except where it thinks

It's an interesting idea. Assuming it's feasible, I wonder whether people would actually want to use a messaging service that's private, except where it thinks it detects illegal activity, in which case messages are copied to the police (and end-to-end encrypted on the way there, of course). That seems practically and logically equivalent to a messaging service which is not end-to-end encrypted but run by a trusted operator.