Hard to do something with flying. I mean can you name an obstacle they can add that doesn't have to do with knocking a player out of the air? They stick with what they can. Hence why Timeless Isle's features made it into the core game. WoW isn't GW2, there's barely any veticality they can create without breaking their game.
Not sure why you're arguing flying should be part of the game. It already is. Every expansion had an obstacle to obtain flying, it was never a one time deal. As for Argus, the Pathfinder achievement was pretty clear about what zones were included. Not to mention the fact that Argus is an instanced zone, something their engine has trouble with ever since Cata (Skywall raid was originally mentioned to have flying) and lack of testing.