The multipart trend has another harmful effect: it encourages authors to "test the waters", and abandon the planned trilogy if the first one doesn't sell well.

The multipart trend has another harmful effect: it encourages authors to "test the waters", and abandon the planned trilogy if the first one doesn't sell well. Since authors are just trying an idea, frequently the first part is a slog where the plot barely advances, then a final action chapter that ends with a cliffhanger. No wonder many never continue.