“Few people go back to content that was trivially easy; most people repeat content they failed the first time.” – Zubon
I believe the above is true on average. Yes, some people rage-quit instantly, while others love nothing more than facerolling something over and over, but I believe the majority of gamers follow the above, be they MMO players or not.
I think this is a major reason why so many themepark MMOs fall into the 3-monther category; players speed through content because they can beat it the first time, and aren’t interested in beating something trivial again so they aren’t as motivated to roll an alt.
WoW somewhat gets away with this because they have so much content, both during the leveling game and then at the level cap. The problem for other game’s isn’t so much that they can’t compete with that, it’s more that the players they do get just hit that wall and leave.