I’ve been thinking about it. When a series of books strings you along after the end of one particular part of the series (say, Harry Potter), they’re doing it to create a plot, to weave a story. They aim to entertain.
When series like Lost strings you along, there is no end in mind. Their aim is to keep you going longer, and get more cash out of you (through advertising, iTunes, rentals, or whatever). When you really consider it, it seems kind of manipulative, doesn’t it? They never really intend to get you to the end– they’ll only do it when the audience is really sick of being strung along. It’s almost like having a really manipulative girlfriend that wants you to buy her gifts, one after another, to stay with her.
Are you really better off this way? You gotta wonder.