We had such thoughts originally, but the player feedback on the first few released DLCs and then the surveys have revealed that our expectations were partly off. A vast majority of our audience is more invested in content that ties into the main campaign, rather than the sidestories. Even if it requires a player to redo the game or reach a certain Act first. I mean, most of you guys redo the game multiple times anyway
While the story of Through the Ashes had to be finished, we've made sure that even the players not interested in the side story had a meaty chunk of content to play with. That was one of the reasons it took so long. I'm certain the wait was worth it though, just look at the new stuff! And there are new armor mythic feats, lots of new spell and environment VFX and Assassin redesign coming with the patch 2.2 together with the DLC, too!
As for future releases (RT in particular) though, it's unlikely we'll be exploring more full sidestories in the nearest time. Expanding the main story experience will be a priority.