Said it many times, the Developers in WoW don't want to put in the effort to develop a zone with three dimensions. That's the only reason for the removal of Flying. Nothing else.
That said, it's a retarded idea and it'll cost them millions of customers and when they finally admit that it's a retarded idea and reverse themselves, they'll make a big spectacle about the 'return' of Flying and sell it as a new feature. Because they're WoW developers.