Blizzard tapped into a capitol resource no other game has discovered. They aren't going to turn off a money engine. Fun is free, go outside and run around inthe sunshine, go for a walk by the river.
You wanna play a video game, you pay and you never stop paying, or the game wouldnt exist.
Paid once for Diablo 2, paid once for LoD. Never paid anything beyond that and that game is still going to this day. Diablo 3 does not need an RMAH to keep the servers going, it's just a side benefit for Blizzard.