After WoW's success it wasn't long before other game studios decided to put out their own MMO to compete with Blizzard's monopoly on MMORPGs. However, by the time these other MMOs came out, World of Warcraft already had already established two competitive edges over their competitors, 'switching costs' and 'intangible assets'.
Switching costs is what keeps customers from leaving one company to another by means that are expensive not in money but rather in time. For example, many consumers do not switch banks because of the extra hassle and paper work that is required for them to transfer their money from one bank to another. The same concept apply for World of Warcraft. Players may spend hundreds of hours into their characters and accounts and may not wish to see themselves do the same thing for another MMORPG, especially one that is new with less content and more bugs.
An intangible asset is a popular brand, a patent, or perhaps a regulatory benefit, that may provide a company with a competitive advantage. Warcraft, is a brand name that dates back to the 1990s, starting with the first Warcraft game, Warcraft Orcs & Humans (1994). Therefore, World of Warcraft had a huge intangible asset from the beginning with its Warcraft series that developed a huge fan base over the next few several years.
As we can see Blizzard has been very successful in the MMORPG industry. Blizzard was able to establish an competitive edge quickly to keep competitors at bay. However, many competitors have been slowly chipping away Blizzard's economic moat, but many may have also have failed to realize that the industry isn't large enough for all the competition that is out there. There will be a time when potential businesses will see that the market is too saturated with MMORPGs and that the best course of action is to stay out of the market than to enter it.
I guess Blizzard saw this and decided to cancel their next MMO project, Titan. But regardless, I am willing to bet that Blizzard knows it has a huge stronghold capable of defending its MMO crown and that maybe true to come. As long as Blizzard manages to keep or improve on its competitive advantage, I don't see them losing its MMO crown.