And when there is nothing left to spend gold on then they'll magically start using another currency? If there's nothing left to buy then they won't buy anything.
Also there are expansions.
If gold is used as the currency in the AH, do you honestly think people will choose manual currency trades over convenience?
BoE and BoP makes no difference, it just means you can buy the best items in the game as opposed to having to pay people to run you through the dungeons that give the best items.