Had delivery estimates slipped people would be complaining that they arent making them fast enough and calling for Tim "supply chain expert" Cook's head. Now because they have enough stock people say they clearly aren't selling well.
The people staying up until 2-3am to order a phone are more commonly the people that want the Xs. This kind of phone is perfect for the person that is looking to upgrade, but doesn't really care about the bells and whistles. Someone walks into the store and sees the colors and the $250 price difference and their mind is made up. The people that REALLY care about OLED, 3D touch, multiple camera lenses, etc are the people that were always going to buy the Xs anyway.
This will sell really well in the long run, imo.