Blizz removed this achievement from the requirements for flying, saying they never intended it to be a real obstacle and that it was only meant to be thematic. I still suspect players were overreacting that first day when they missed the first invasion, and that it would not have mattered for 99% of players if it had remained one of the requirements. We have to get to Revered with Legionfall anyway, right? And regardless of how much time you spend in game, the latter seems to take much more time than the invasions. Blizz has done so many of these world events, dating back to WotLK and Cataclysm, that I figured all along the invasions would rotate and be numerous enough we'd all get plenty of chances.
My main has "Defender of the Broken Isles", but is still 9k/12k for Revered. How about you?