There is an alternate explanation for why Flying via Pathfinder is being delayed so long. Granted, it's a bit of a long shot, but within the realm of possibility:
Blizzard is delaying the release of flying in Legion so long because they new areas becoming available in 7.2 and onward are being built from the ground up(if you'll forgive the pun) to include flying instead of treating it like it doesn't exist. This would, of course, make development of the areas in 7.2 and later take longer, but would also mean that flying would not be the content-trivializing phantom menace that everyone seems to think it is.
Anyway, when you look at all the tools Blizzard gives us right now to skip content, it's not like flying is actually going to hurt anything. Just make quests "Kill 11 boars" instead of "Kill 10" and you get the same result as restricting flying creates.