The notion that all but the whiniest of players will quit if they can't fly from 91-100 is nonsensical. Or else they would have quit when going from having flight in TBC at 70, to not flying in Northrend till 77. Or from flying in Cataclysm in 85 and then not flying again till 90.
Furthermore, many games make a point of granting you a convenience ability in certain areas, then removing it in later areas to add an additional challenge.