It's not like there is no upside! I would never have leveld up archeology without flying mounts in the old world. Actually most profession leveling or achievement hunting would be terrible without that.
I think the problem that flying mounts pose in world pvp in general is just especially obvious in the old world because most ganks there will be 90s on flying mounts vs lowbies on ground mounts.
I'd gladly get some actual pvp going with some gankers but the sole reason the ganking is so prevalent (which before flying mounts was a great way to get some REALLY huge battles going) is because there are no repercussions. I might catch them in the act once but it only takes 1 gcd worth of time to mount up and "unflag pvp" after you rez so it really is a futile endeaver. I cant even be arsed to hang around the dark portal because all that ever happens are flying mount standoffs between 90s....