All you're doing is applying your current knowledge of WoW as it is right now, and taking flying mounts away. That won't work, because current WoW is designed with flying mounts in mind.
Imagine if it was designed without flying mounts. Imagine what could be implemented if they didn't exist. There would be plenty of changes to the game to accommodate this change.
There is nothing that flying mounts provide beyond convenience and emotional attachment. Nothing. Yet if they were removed, we'd gain so much more.