No one is arguing about flight during leveling. That’s not an issue. The issue is patches and zones dropped for play after an expansion’s launch.
There’s been patches dropped after expansion launch that introduced new content that allowed flight. As well, there’s even been max-level zones for players with relevant content, such as in Wrath, that were built with flight in mind.
This idea that “all relevant content must disallow flight” didn’t come about until the development of Pathfinder in Warlords. Because they wanted a justification for withholding flight until later in an expansion’s lifespan.
This was a new justification to somehow promote the idea that daily quests and reputation grinds couldn’t survive contact with flight. That flight could no longer just be the reward for reaching max-level. And on its face, it’s ridiculous. Because we’d already had daily quests and flight existing together for years.