I've read both James Comey's autobiography and Andrew McCabe's (former FBI Director and Deputy Director, respectively). Both of them go into some depth about the trouble of end-to-end encryption when it falls into the wrong hands, but neither one of them seems to get why people use it. I doubt anyone uses encryption as a form of "to make sure the FBI can't get in."
Law enforcement on every level feels like it's an undue hardship, when the reality is, when you make the populace terrified of their stuff being stolen, misappropriated, or otherwise accessed by both illegitimate tracking means (which is to say apps that want to monitor your smartphone at all times without disclosure, which should be a capital offense - or apps that are datamining everything you do, which should result in huge civil fines per infraction to the point those companies are bankrupt and their leadership penniless for the rest of their days) or by intrusion, why wouldn't everyone encrypt everything all the time to prevent this?
Almost no one's world revolves around keeping the FBI out of their phone. It revolves around protecting what little information we can. The FBI's inability to access information is caught in the crossfire, and most people don't particularly care. Encryption's been in use for decades in the form of ciphers, this is just one created with far more complexity and digitally. If encryption wasn't widely used already, does anyone think people looking to do awful things wouldn't come up with an alternative? Is the FBI that dumb? Or is this just a reductive argument, whining about things they don't like? If they couldn't do it one way, they'd do it another. Hell, "the wedding" is coded language in FBI investigations for potential attacks.