If that were true, TBC and WoTLK would have posted different sub numbers and never reached 12 + million active gamers. So, your idea doesn't really hold water.You are making valid points, I understand what you are saying, but the problem with flying is it removes too much content. Its not good to have the world being interacted with purely on the players terms dropping in and out of the game.
IT wasn't until the quality story, content and general lack of content did subs fall. Cataclysm and MoP had problems but flying wasn't one of them.