You can measure the success of GW2 based on how its expansion will sell cause the vanilla GW2 was hyped as shit. I too don't think that the expansion will sell so much based on the comments that people make in the videogame websites when Anet introduces GW2's monthly updates, most people say that they have nothing to look forward from GW2 since it was only entertaining for a month and then it got boring really fast which i agree. I feel that Anet wanted to create something for everyone and in their attempt they created a jack of all trades but a master of none.
GW2 expansion sales will show us if its a success or a failure now that people know what kind of game GW2 is.