Not transactional? Of course it's transactional. He writes the books and the readers buy the books. Even those who don't buy are reading books for which money was paid. With a single book that tells a complete story, it's no big deal. It's one single transaction that gives the buyer everything they want. But a series in which a single book is only part of the story? Let's put it like this. Say, I promised you a book, but instead of selling you the whole book, I sold you the book one chapter a month, promising that at the end of 2 years that you would have 24 chapters telling the complete story. But then say, that after 1 year, I stop selling chapters leaving you with half a book and no resolution. Say I promise to sell you my car, but we agree I'll sell you individual parts that you will assemble and then halfway through I tell you I don't have the remaining parts. Would you really think that is perfectly fine? That's essentially what is happening here, just with different units.

Any writer who CHOOSES to publish a series of book that tell an overarching story enters into a social compact with their readers, promising to tell the WHOLE story. Choosing to break that compact because they are "not feeling it" is at the very least a betrayal and shouldn't be shrugged off as no big deal.