I believe a BIG part of Runescape's success is based on it's accessibility. It's the top of browser games in terms of quality and gives it some reputation as well. Plus, the developers at Jagex right now are familiar with this engine, this kind of development - I don't think making it client based would be better for Runescape. Yeah, but compare it with another java-based game. Say, minecraft for example. Both available for download, and able to be run in browser. You can play Minecraft in your browser now?