He may not think that end state is "sustainable"
But he has no choice. Even if the US passed draconian laws requiring key escrow, nothing stops terrorists from using software designed in other countries that aren't subject to those laws. If they made Google & Apple ban those apps, terrorists would sideload/jailbreak, or run Google free versions of Android that Google can't handicap.
The FBI is going to have to learn to deal with terrorists having unbreakable encryption, that's the reality and nothing is going to change it.