Maybe if somebody jumps on their flying mounts after you have ganked them that should give you a clue that they cant be arsed fighting with you. and before people start slabbering on about PvP servers people had ways of denying you your "World PvP" way before flying mounts such as ressing as far as possible from you and gtfo on their ground mount or just logging out for ten minutes. Sure, flying mounts are as convenient way as any of getting out of a fight but no matter what "reasonable restrictions on flying mounts" you place players will always have a way of NOT World PvPing. Flying mounts have next to nothing to do with the death of World PvP; put out some rewards for player kills out in the world which are not available through any other means and you will see your resurgence in World PvP flying mounts be damned.