Unless you level primarily using dungeons. Then what? You'll have flying in zones where most people don't.
This is why flying probably should have an entire line of quests, at or near max level, that not only explains the lore and RP reasons for why flight isn't available and how we get it back, but also to control the actual spot that players obtain it in the content release schedule. Think of it in terms of something like getting your other artifacts in Legion, or possible more along the lines of the Garrison campaign quests.
This is also what bothers me most: Blizzard has time to create campaign quests for each of 12 classes, AND time to create quests for all of the crafting professions, but can't be bothered to explain flying, which virtually everyone is going to use?
- - - Updated - - -
Except that the value of flight as a "reward" is completely out of parity with the amount of time and effort required. You only get flying once all the content is exhausted.
Great! So we've got this really powerful tool for getting around content that we've already seen hundreds, if not thousands, of times. :/ Unless 7.3 and further has content I can fly around in when I'm doing it for the first time, flying is about as rewarding as $1 ramen noodles you left out over night to coagulate.