I think it's a cool bit of trivia. And frankly, had things happened as they were originally written, would we really consider Darth Vader to be anywhere near as threatening of a villain had he just let Obi-Wan escape? Clearly this was an instance where the re-write improved the story dramatically.
I'd play devil's advocate and really sink my teeth into the scene, here...because I think it could have used another rewrite if the point was to add menace to Darth Vader and the whole Death Star encounter as Lucas explained it.
Vader doesn't win the duel. Ben stands there in a non-defensive position while Vader swings his lightsaber to cut him down. But, before he connects, Ben Kenobi "disappears." Then we hear Ben's disembodied voice instructing Luke to run 2 seconds later. It is clear to the audience Ben didn't die in the commonly understood sense.
The last we see of Vader during the escape he is stomping on Ben's empty clothes cluelessly. He clearly has no idea what is going on, and certainly doesn't feel victorious. He looks like a chump.
Ben sacrifices his corporeal body in a tactic to keep Vader and the stormtroopers occupied while the kids escape. More weight could have been given to his death had it been written so he was trying to win the duel and had been overwhelmed by the stormtroopers joining the fight. But that's not what happened. He pulled a Jedi magic trick that never even looked like he got cut down (except to Luke, who grieves for him on the Falcon for a couple seconds before the TIE battle.)
It is a better story point than Ben making it back to the Falcon as if he had just been toying with Vader. It does take care of his uselessness for the rest of the movie (and led to the introduction of Yoda in ESB, which was a great character and great concept in and of itself.) But I can't honestly say I ever felt Vader won the duel.
The way you've analyzed this is very much from a standpoint of hindsight. But put yourself in that theater seeing the film for the first time. Because the scene plays out quite differently when you look at it from the standpoint of a fresh viewer who doesn't know that Obi-Wan has been studying the Force, and how to maintain his identity.
As for the duel? Ben is having a hard time holding his own. Both he and Vader are not in top dueling form, and Ben has largely been on the defensive while Vader has been the aggressor. The only indication we have that the duel is taking a toll on Vader is that his rate of breathing has increased. Then he sees Luke and the others trying to escape. He knows that giving himself up and the distraction that may cause Vader might be the only way to ensure that Luke and Leia can escape.
When Vader strikes Ben down? To the audience it comes across as a cold-blooded murder. Ben stands there defenseless, having given himself up. It makes Vader seem that much more sinister to the audience to have killed an opponent who has effectively surrendered. While there's clearly more to the story, we're talking about audience perception in the moment. We don't know until much later that Vader played into Obi-Wan's hands, and that Obi-Wan had been preparing himself for a moment like that for nearly 20 years.