I suppose the formula works because of the amount of readership it gets. I mean, if it's the most appealing for majority of her audience, why not? There's a market to be sold. However, this does not mean I approve sacrificing novelty for the sake of what the majority wants. Sometimes it's not on the author's side but the publisher's wishes that get granted and the story gets distorted because demand wants it to.

I personally have not read the books you mentioned but your analysis is insightful in helping me analyze a lot of novel series I patronize.