John, Goosebumps was a huge series of books that went on for a long time. Harry Potter was spread out over ages. The Twilight series is done; no more books, Meyer is moving on to other vapid trash (There was supposed to be a version of the first book done in Edward's perspective, but the first 12 chapters got leaked online and she chucked the shits and put in on hold indefinitely). The only way the series gets any bigger will be from the movies, but even then I am pretty sure people will forget about it all pretty quickly. It's not different enough to keep people interested, and not well written enough to be an average story that stays popular.
I do have a big issue with the characterisation. Bella is a terribly pathetic character; you don't feel all that interested in her throughout the books and it's mostly in her perspective. In fact, none of them are particularly engaging characters, because there's not that much description of them. Bella is clumsy, loves Edward (Oh no wait, she loves Jacob too, scandal!), wants to be a vampire. That's it. Oh wait, she smells delicious and everyone wants to kill her. Edward loves Bella, is crazy overprotective, doesn't want to turn her into a vampire, and is GORGEOUS. Wow, great couple there. It's just very shallow; Meyer's writing has kind of gotten better over time, but Host has the same kind of pathetic main character (and goddamn love triangle) as this, so maybe she just likes having weak lady protagonists?
It really does read like a teenage version of a penny dreadful. I have read a fair amount of those kind of books and it has been much better. I've read worse, but don't get why people obsess over them. The only good thing about having such little characterisation in the book means the readers can create whatever kind of fantasy they want and it won't seem that out of the ordinary. Hmmm, I wonder what kind of fanfics there are out there...