The real truth is that George killed Obi-wan as a plot device rather than keep him alive as he had nothing to do in the third act of Star Wars.
But he managed to put Obi-wan in the cockpit with Luke so it was not a bad decision really. As the Saga progressed it got more messy.
But it created a nice mythology. Sorry about that…
That begs an interesting question, did they shoot Luke’s scenes in the X-Wing before or after they made the decision to kill off Obi Wan? Could they have shot any scenes with Ben post Death Star?