Avatars take up space. If a world you have a certain amount of entertaining content in it, that content will almost always be subject to some kind of congestion effect. The cool monsters are in the Dungeon of Befallen, but if tens of thousands of us go there to hunt them, none of us will have a good time. Sometimes the only way to reduce congestion is to add content, but this, again, is labour intensive. There will also be effects related to connection speeds and bandwidth congestion.
A third reason that the market will probably not be dominated by a few companies can be found in the many competitive strategies that are available even now, but have not yet been exploited by new entrants. For example, the current set of developers have managed to impose huge switching costs on players by structuring gameplay around the time-intensive development of avatar capital. A player starts the game with a weak avatar, but gameplay gives the avatar ever-increasing powers. As power increases, the avatar is able to take more advantage of the game world, to travel farther, do more things, see more people. A person with a high-level avatar then faces a high hurdle in switching games, because in the new game he will start out poor, defenceless and alone again. This situation definitely locks in the game s player base, but it is also open to defeat by any number of schemes to reduce the switching costs. Surprisingly, no competitor to a current game has offered new players the opportunity to start their avatars at a higher level of wealth and ability if they can provide evidence of a high level avatar in another game. On the other hand, two games (Ultima Online and Dark Ages of Camelot) now offer methods to effectively start out ahead: in Ultima, you can buy your levels directamente; in Camelot, you can start a new avatar at level 20 if you have already gotten to level one 50.