Ramin Shokrizade has an answer just for you. A virtual world economist, Shokrizade states that up until EVE Online and World of Warcraft, games grew in size and scope and then stopped. Why? Because it was never about getting big; it was about protecting and ensuring the equity of the gamers.
Shokrizade delves into why equity -- the sum of gamers' in-game efforts, such as levels and possessions -- is so important in a game and why those games that ignore protecting this equity don't measure up when it comes to success. He discusses how microtransactions that deal in in-game content destroy equity as well as how expansions that make previous content (such as crafting tiers) obsolete do the same thing. He then goes on to discuss the games that get it right, like League of Legends and World of Tanks.