You are forgetting about the structure of the MMO in your analysis. If ever a player is to leave their current MMO, they must somehow rationalize that they are giving up 6+ months of time they invested in their current character.
This is very easy to do if you are a veteran MMO player (as in, someone who has played since the start of the MMO craze - 1999-2001 era) because you were forced to do it with your first character when your favourite game went out of business, or you sold your character on ebay. Once you rationalize it the first time, its very easy to switch games in the future and give up your characters.
However, for a new MMO player (I'd wager this is around 90% of WoW's player base), its very difficult to rationalize. There is no more interest in buying characters on ebay due to powerleveling, and WoW will not die for a few years. This leaves the player with no outside forces to help rationalize, thus they never feel it worthwhile to leave. The only time they will leave is if their guild (led by a veteran player, perhaps), also leaves.
That reason, and that reason alone, has solidified every game since the beginning of the genre's existence. WoW has particularly benefited from this because it has very strong branding and marketing, thus capturing most of the MMO-virgin player market. The same forces helped sustain the MMO pioneers: EQ, Lineage, Ragnarok Online, etc... until they came out with sequels.
Once a company introduces a sequel, they are inadvertently forcing their players to rationalize the decision to switch games (to their sequel). However, once this decision is reached then the flood gates open, and the player explores the entire list of available MMOs before committing to the sequel; often realizing that there has been other games all along that were better suited for their playing style. Thus I predict that the introduction of WoW2 (or whatever game Blizzard is marking) will be the end for WoW and very few players will continue on in the sequel.
Once the new players do this, they will never again be "strapped" down into an MMO. After a few iterations of this (I gather that we are reaching saturation for the market soon enough), MMOs will be populated based on their enjoyment and gameplay, not on their marketing. Finally, an era of innovation will come upon us.
For those veteran MMO readers, you know that this has already happened in Asia. Once Lineage and RO lost their hold on the initial player base, the genre exploded into a vast array of innovative games. This also rebutes the parent's comment: the current North American MMO monoculture sucks - the Asian MMO culture has been experimental for 2-3 years now.