Susan Landau's congressional testimony - with Comey siting one table away - applies to this as well. She explains in detail not only the problem with backdoors, but also how the FBI badly needs to update their methods. The quote from the NSA that legal access doesn't mean that access will be easy should have ended this brouhaha last year.

https://www.youtube.com/watch?v=g1GgnbN9oNw&t=3h35m50s