A feature like this REALLY should have been ready by launch.
Also, I saw someone in another thread make a terrific point -- lest we not forget the iPhone 7 launch. A promised big feature on the Plus model didn't arrive for a long time after launch -- portrait mode.
Honestly, that isn't a very good point. To compare a flagship feature like a voice assistant to a relatively minor one that only 7 Plus users can use is apples to oranges. More than that, Portrait Mode was available in beta on day one and available for all a few weeks after that.