Anakin vs. Obi-Wan has no real bearing on the story at all. Neither one of them is killed, and it's not where Anakin makes his choice, and even if Anakin was killed it wouldnt have mattered because Sidious is the problem NOT Anakin. This fight is more "symbolic" than anything. It's symbolic of a relationship gone bad more than a primary piece of the story. The place where Anakin makes his choice is during the Mace/Sidious duel. The fate of galaxy was to be determined during the Sidious/Yoda duel. Showing how manipulative Sidious can be was the reason for the Anakin/Dooku duel. Besides the breakdown of a friendship, Obi-Wan vs. Anakin really only has one thing going for it....it shows us why Vader needs a breathing machine, but to say it's a pivotal part of the story isnt true.