Let's be honest here. People wouldn't quit the game over it. They'd bitch and moan or a few days, but after people are buying it and it becomes the "norm" in game after a week or two, players will just accept it.
It happened with the sparkly horse, pets, and now the helms. History will repeat itself, as in people bitch for a few days and then noone cares. The same will happen when the xp bonus and lesser coins become a buyable item as well. People won't just quit the game they've invested 5+ years into just because Blizz is selling models that you don't even need which won't affect your game. Blizz knows this as well which is why like I said, it would be dumb not to sell models in the cash shop.