Yeah they do, loads of people. Ever wonder what the other 50% of the MMO market are playing? :P
It is mainly because WoW has little to no challenge (ghostly corpse runs are a terrible idea, imo). I can also tell you how illegitimate that player base is.
WoW playerbase: about 6.5 million accounts.
.5 Million are chinese farmers
2 million are the gamers just looking for something to play in Japan/Korea/China (countries where they have Lineage 2 and WoW and that is just about it for MMOs)
1 million are the hardcore gaming wannabes that only play WoW because getting to hear a guy on teamspeak scream out strats make them feel special and epic, like a character in an FPS, while they just sit there and ignore real life
1 million are the WoW kiddies (who have never touched any games other than Halo on their XBoX) who f**k up the game's community
.5 million are the second (or third) account of players who like to have multiple PCs on the game at once
1 million are Warcraft fanatics that are biased against anything other than the work of their precious Blizzard, who I must admit did create some pretty awesome RTS games in the company's time
.5 million just decided to jump in randomly because they judged the quality of the game by the numbers of subscribers
The real MMOers are into games like EQ and DAOC, FFXI (which actually kind of sucked), COH/COV, and the older MMO games that have alot of meat on their bones. And, alot of the main playerbase for the genre tried WoW and was sickened by the repetitive simplicity of the game. Ultimately, WoW has 15 million accounts, while about 6.5 million are active.
Back on topic, a Mario MMO would be kind of cool. Although, a Zelda MMO could really be a winner.