It's probably about 20 or 30 pages back at this point, but I believe it was Phumbles who said it.
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For me it's pretty simple.
I do not want to contribute to box sales and subs while flying is completely unavailable. I don't want to reward Blizzard with my money when the present state of the game is not acceptable to me. The only vote I have is with my wallet. I am not willing to buy the expansion and then stay subbed for months during which flying is not available. Literally the ONLY thing I am willing to do without flying, in the world, in Legion, is the achievement to get flying. That's it. I won't do pet battles without flying. I won't do archeology without flying. I won't collect toys without flying. I won't do optional world achievements without flying. So if I don't want to sub for months of no flying, and there's nothing in game I feel like doing without flying, why on earth would I buy the game now? When I can skip months of frustration and save myself some money?
I'll buy Legion and start the achievement when flying is on the PTR and not a moment sooner. I played the beta and didn't see a single thing that would make me want to be subscribed while flying isn't obtainable.