He will publish targaryen stories precisely because of that. They are more important. The main story is awesome, but it doesn't showcase the world nearly as much as other time periods and characters being introduced. He wants to make a legacy, not to please fans, just like Tolkien. Tolkien's main story books that we know as TLOR were not his focus, the focus was on the side stories, the creation of a world and its' various cultures, characters and differences from ours.
He'd be one hell of an idiot to focus on the main story, it has already explored the world of its' time to the max, he can't add much to it, where focusing on a different time period could give a whole new picture of the world.