That's true at a point in time, but bad guys start out as clueless noobs with poor opsec. The Silk Road guy, for example, was identified by forum posts he made

That's true at a point in time, but bad guys start out as clueless noobs with poor opsec. The Silk Road guy, for example, was identified by forum posts he made before becoming a drug lord. The sort of people who become radicalized through online videos aren't using strong crypto until after they've committed to becoming terrorists. So a database of texts going back several years is quite useful in catching actual bad guys. Which is not to say I approve of more surveillance. Just that surveillance of convenient modes of communication (iMessage) is useful to serious crime fighting.