My whole beef with “preserving the twist” is that we never see Obi-Wan training any other pupils in the PT. Even if we did, when he tells Luke that Darth Vader was a pupil of his that fell to the dark side, who could he be talking about except the pupil we followed for three movies that obviously at least flirted with darkness?
What’s more reasonable to assume, that he had another pupil we never saw or heard him mention that also happened to fall to the dark side, or that he’s lying about it?