Re: MMO
Not necessarily; the point of MMOs is to get you to play as long as you can, increasing the chances of you using real money to get items if it is a free MMO, or making you continue to pay the monthly fee, if it is a pay-to-play game (or both, in some cases...). They do this by either doing what WoW and only WoW did, which is make the game so diverse, unique and fun, you never want to quit playing. Leveling in WoW is not nearly as hard as some other games (Ragnarok Online), but once you're level 80, what's next? Get all the Tier sets.
Other games, such as MapleStory (which has bombared the United States with advertisements) take an extremely long time to reach the maximum level; there is no appeal to the game except pixels you can buy with real money, and getting your friends to play so you play. And it's successful.
The special thing about single player RPGs is that it's usually easier in the aspect of levelling, gold and looking like how you want to. In WoW, it's quite hard to get Tier sets; in MapleStory, it's extremely hard to get to level 200. In Fable, although we all criticize the game for its difficulty level, it's one of the things that defines it (although upping the difficulty a bit wouldn't be detrimental at all).