You keep using that word. I don't think it means what you think it means.
WoW has had substantial competition since before it launched. Yes, none of them are as successful as WoW, but they are still successful.
If WoW had a monopoly then any and all of its competition would be shutting down and closing their doors. This doesn't happen to very many MMOs at all, really, and those that it does happen to usually had a very bad start and saw little improvement.