I think it works because George Lucas created a compelling drama that was so much larger than just the question of Vadar's origin. Coming into the prequels, all we knew from the original trilogy was that Anakin had been trained by Obi-Wan, but had been seduced to the dark side by the emperor. We didn't really know what that seduction entailed. I also found the world building in the prequels to be thrilling. It really depends on how Rowling structures the future installments of Fantastic Beasts. If the structure is all about leading into a final battle where the outcome isn't in doubt, but the movies are built around a hook of "what will happen when good and evil finally meet?" that's just not very interesting dramatically. If the question that drives the narrative forward is about something different, then it'll play much differently.