A world is made of multiple servers. They might not be able to simply add more servers to the world to increase performance/capacity, it depends on the software they're using. Replacing the servers with faster/larger hardware may not be an option. So basically, adding more worlds is the best thing they can do at the moment unless they can add more servers to the world. They should really give people free character transfers to make use of the new worlds. Even better would be if your characters were kept on a central server and you weren't locked into always playing on the same server.