Back in 1977, no one had seen what a lightsaber battle before, so the showdown between Obi-Wan Kenobi and Darth Vader was quite the spectacle to behold. Unfortunately, subsequent years of flashier and more intricate lightsaber duels have diminished the impact of this initial fight, and when you factor in what we now know about both characters, the epic final confrontation between Obi-Wan and Vader looks more like two senior citizens hitting each other with their walkers than a clash between two titans.
While there are plenty of perfectly reasonable explanations for why the scene is so lackluster, such as the fact that Star Wars was essentially a big-budget indie and no one knew what a lightsaber fight was supposed to look like at the time, there could also be a canonical reason for why the fight is so lame in hindsight.
As we’ve seen recently in Rogue One: A Star Wars Story, Vader is still a force to be reckoned with at the time A New Hope is set. In his attempt to retrieve the stolen Death Star plans, Vader displays his true power, suggesting that he was holding back in his confrontation with Obi-Wan, which takes place mere days or possibly weeks after the Rogue One scene. Vader easily could have been more aggressive with his former master, but he wasn’t, which suggests that he still holds affection for his former master.
Of course, we could also point to Obi-Wan’s feebleness as the reason for Vader’s show of mercy. “Your powers are weak, old man,” Vader tells Obi-Wan mid-duel, but the most recent episode of Star Wars Rebels shows an Obi-Wan who is anything but. In the episode, Darth Maul tracks down Obi-Wan to his home on Tatooine to finally exact his revenge for his defeat at the end of The Phantom Menace, but Obi-Wan is able to defeat Maul without breaking a sweat. While Star Wars Rebels is set a few years prior to A New Hope, it seems unlikely that Obi-Wan would go from a fully intact Jedi Master to a barely capable old man in such a short time frame, which suggests that rather than being weak, Kenobi was holding back against Vader as well.
This information paints the Death Star fight in A New Hope in a whole new light. Instead of two old men who look like they barely know how to hold a lightsaber, we see two former friends who are reluctant to strike each other down because of their years of pent-up grief. It’s only when Obi-Wan realizes that he needs to ensure that Luke escapes that the old Jedi is able to sacrifice himself to Vader’s blade, ensuring that the younger Skywalker lives to fight another day and potentially bring balance to the Force at last.