I think they haven’t thought enough effects of both postponing flight and making people do so many things to get it. Problem lies in the fact that flying used to be a thing you got with gold when you were high enough level (in Cata even from very start of the expansion). It was not reward, it was tool that made the game more fun. So first issue is that many people feel like something was given to them, then taken away and then being hold hostage untill you jump trough all the hoops. Not the nicest feeling and not giving very good image of company whose whole purpose is to entertain you.
Second problem lies in how pathfinder is made. Idea of exploring everything and doing, well almost everything might sound good at start but what it causes is people not being able to pick and play things they wish but are guided to achievement tube they must do in order to get flying and when they are done, they have already done and seen everything so even they have got flying they have same time exhausted all the content (same content they originally would have enjoyed so much more with flight on the first play through).
Situation is little bit like if someone would go for years to a restaurant that makes amazing pasta carbonara to eat it and then they would start to demand that you first have to eat 4 plates of plain pasta before you can order that carbonara. So you really like that carbonara and you eat those plates of plain pasta and when you get that carbonara finally front of you it does look delicious but you just cannot stomach eating any more pasta.
Ok now i got hungry.