I really don't understand why the success of a setting has such a huge impact on whether it returns or not. At the end of the day, the cards dictate the success of a block. Sure, flavor is great and all, but it's not like Return to Ravnica had any leg up on quality or that a return to Kamigawa would have a harder starting point to work from. They've got to come up with interesting, playable, and new feeling cards regardless of the setting they use; I'm not going to be optimistic or pessimistic based on how many of those they happened to create for the last block in the setting.
When a block is successful, people tend to remember it fondly and that is a natural boost for inital sales of the return set. It's as simple as that.