Lineage 2 was successful in fact they are doing another sequel and Final Fantasy while not a true sequel is still a sequel to Final Fantasy 11 and makes a fuckload more than the original. Everquest 2 was successful enough that they have been doing expansions for it for the last what 15 years? Same with Adventure Quest worlds. Wakfu still seems to be updating.
If you look at the list of failed mmos very few of them are sequels. The only one I can think of offhand is Asheron's Call 2.