Okay, I see your point.
Maybe it's partly the fault of marketing? They think they'll sell more copies if they emphasise this as a the real end of the series, or something. I'm sure it won't read like a proper ending... its in the nature of the series to be always in development, always expanding. It's part six of three.
I don't like all the Colfer-hate, because he really is a brilliant and hilarious writer... and if Douglas Adam's widow said to you, "want to write a new Hitchiker's book?" would you say no?
Also, it's interesting how you say that this book is "unnecessary and unwanted" by the fans. I mean interesting in a genuine way. It reminds me of the anger of some fans when J.K. announced that Dumbledore was gay. A lot of them thought she was plum wrong. The question is, who owns the story? Who defines meaning? Is it the author, the reader, or the text itself? I personally think of texts having a life of their own, which is another reason why I'm not angry about this new book. It's like the series wants to be told some more, and its having to make with a new storyteller in order to do that.