Cash shop would work, but people will complain about the "achievement" of the game being gone. What's the point of earning items when you can just buy them. And especially with this game, achievement is one of the most important things to keep it interesting. And most gameplay isn't the that fun, sometimes it feels like a chore. With the games you listed, Ice, gameplay is decently fast paced, and/or pvp oriented, which helps the fastpacedness. Those games are less about the eventual long term goal, like RuneScape (camping a boss for money, skilling, getting quest requirements) and more about the instant gratification of winning or successfully. The cash shop eliminates any point in achieving things, so RuneScape would have to drastically change how it's played to make it fit in. And RuneScape is generally pretty casual; there won't be a lot of customers.