The "just use ground mounts" argument is as laughable as telling people who think leveling is too fast not to use heirlooms, well not using them feels like garbage because you lose 50% xp (ironically) and they have far worse gear in the process. Almost no one are willing to gimp themselves even though they would prefer the alternative.
So why would anyone force themselves to take extra risks (daze, pvp, falldamage), travel about half as slow, when they can just jump on a flying anyway.
And that's why flying is going away for now. Too much convenience is detrimental to the game.