Let me try to clear things up a bit, since I realize a single achievement (Guide des îles Brisées, première partie
) out of context isn't exactly informative.
Overall, this is consistent with the plans we announced last year when we added the original Draenor Pathfinder achievement in 6.2.2, and hopefully does not come as a surprise. The underlying philosophy here is: explore and master the outdoor world on the ground, in order to unlock the ability to soar above it freely. This time around, since we have our design in place from the start of the expansion, we want to give players as much advance notice as possible, and allow everyone who cares deeply about unlocking flight to understand the requirements and begin progressing towards them immediately.
I can't give a detailed estimate of exactly when the next step will unlock. We have a solid patch plan in the works, but we need to remain flexible to adapt to player needs once the expansion launches, and there's no way to even hint at a date without it sounding like a promise. That said, I can say that Part One should represent a significant majority of the total effort required to ultimately unlock flight. Patch content will see players adventuring within new parts of the Broken Isles, and there will likely be one or two additional criteria associated with that content. At that point, players will unlock the ability to fly throughout the Broken Isles.
Finally, I've seen some concern about rep "grinding" associated with this achievement. All of the referenced reputations are earned by doing level-up quests and world quests associated with those factions throughout the Broken Isles. There are no mob-grinding components to any of these reputations. Frankly, the Legion Pathfinder achievement was much easier to design than Draenor Pathfinder was, simply because we have much more outdoor world content in Legion.