I've always assumed that iOS doesn't do live turn-by-turn directions in their native maps app in part because of licensing terms encumbering the routing data. Moving away from Google to their own algorithms (or to direct deals with other data providers) could free them of those terms. If so, I'd be surprised if they didn't add turn-by-turn directions to the app.

Of course, they'd undercut all the paid turn-by-turn apps, but they haven't been shy about that sort of thing in the past.