The open world currently is not designed to by dynamic or engaging sadly. Lack of flight would do little to engage players or imerse them when the zones are vastly empty wastelands content wise and challenge wise. If they designed the game around world traversal then flying would be a big blow to imersion.

But you can't be imersed in a world that doesn't have it. Getting around is a teleport and a few minutes away no mater what your doing, hopefully the next expansion introduces underground dungeons zones or something where they can actually design for interesting areas for exploration that don't allow mounts or flight.