...and not the other way round.
This is a controversial topic with regards to some issues, but I'm firmly glued to my seat on this one. Authors truly appreciate (and some of us desperately rely) on the wonderful way readers spread the word, but it is not the reader's responsibility to help us build and establish our careers.
I would never imply that an author should sell their soul with their book, but yes, I do believe that when a reader buys a book (and in some cases, many books) in a series, they're also buying into the promise of a satisfactory ending at some point and a reasonable expectation of the author finishing the series. The same can be said for TV series, but that's another topic all together.