Mr Comey and his like-minded counterparts in 'law enforcement' and government around the world are starting from the premise that this is possible. Thus, the conversation they want to have - however 'adult' - is for people to tell them how to do it.
An 'adult' conversation isn't enough; what is needed is an open and honest conversation - one that starts with a proposition for a desired end result and then investigates whether this end result is feasible or even possible before it gets any further. That conversation must have, as a fundamental understanding, the admission from Comey (et al) that it really might not be possible to do what he wants.
Comney and his ilk are trying to jump straight to the 'how' without wishing to seriously address the 'if' first. "Stop telling me it can't be done and start telling me how it can be done."
In that way, he's almost like the two wireless power 'start-ups' getting press: uBeam and Energous. Both of these companies have a 'vision' of an outcome: wireless power transmission that is safe, affordable and efficient. The basic technology - power transmission via sound or radio waves - is certainly possible and uncontroversial. The problem is that to get it to work in any useful way is either dreadfully dangerous or insanely wasteful/expensive.
Similarly, Comeys 'vision' of encryption that can be broken by the feds but not by anyone else has, at its heart, a (technically) uncontroversial reality: it is possible to make encryption that is able to be decrypted by a third party. The problem comes, as it does with uBeam and Energous, when it is asserted that this access can be achieved while keeping all the benefits provided by the current technology.
uBeam and Energous can create transmitters to supply power to devices wirelessly but they are only able to do so by throwing out all the benefits of the existing technology - efficiency, speed and economy - because, you know, physics.
In Comey's case, encryption can be created that is crackable by feds but only if they throw out the benefit provided by current encrytion: security. Because, you know, mathematics.
For both, the focus on the vision renders the product useless for it's main purpose: charging devices in uBeam/Erergous's case and security data in Comey's.