Everyone hates the leveling treadmill, but the business strategy makes sense: Once people spend 800 hours to get to the great content of your game, they will be less willing to spend another 800 hours to get the great content of your competitor's game. So long as every game has a leveling treadmill, each company has considerable pricing power over its installed base. You usually don't see this in the form of higher fees (I am not sure why not), but rather in the form of cost-cutting (minimal customer service, buggy expansion packs, the developer-is-always-right syndrome*). The competitive response is, of course, to cut the switching costs. One way would be to lessen the leveling treadmill or make its effect on the fun of gameplay minimal. Another would be to allow people with powerful characters in your competitor's game to enter your game with a similarly buff toon. A step in that direction: Playvault's migration service (mentioned in this comment by Joel Hutson first), through which a company can set an explicit exchange rate between a competitor's currency and their own. The service is expanding, apparently. If you are a millionaire in Ultima, you can now switch to Horizons and still be rich. Thanks Andres for the tip.

* Of course, everyone at Terra Nova knows that the developer IS always right.