Imagine having the mount equivalent of Twilight: a nocturnal animal that flies (permanently) at night. There would then also be a mount like Sunrise: able to take flight during the day but sleeps at night. Now imagine breeding the two (like Eternity) into a mount that flies at any time. You may argue that it takes away any challenge in the open world, but anyone who puts the grind into these legendary mounts — which should be harder to acquire than weapons, as they are useful rather than just cosmetic — has already done their share of arduous tasks.
Griffon was great during PoF, just like gliding was great in HoT. But I think it is now time for a mount with permanent flying. At least, for the most hardcore of players, who are already bemoaning the lack of content made for them.