What about all the conversations the terrorist had with his fellow terrorists in person? Is it an awful, terrible, no good thing that the FBI can't retrieve those conversations? Should we put in place ubiquitous video and audio surveillance in every square foot of the country just in case the FBI ever wants to review something that happened?
It's not "going dark" that is an unprecedented change, it was the brief "going light" period that preceded it. Law enforcement has apparently lost the ability to do on the ground investigation work in favor of whiz bang-ery (and intelligence has lost the ability to do humint). Well, whiz bang-ery is a two way street. Now is the time to revive those old skills and deal with a _return_ to a world where you can't just outsource your job to a wire tap.