They're all watching, Luke Skywalker, the Stormtroopers, everyone. Darth Vader battles Obi-Wan Kenobi. It's a moment from another lifetime. Kenobi seems to realize that it's not even about him, but a chance for his young charge to escape along with his friends from the Death Star and bring the stolen plans for the weapon to the Rebel Alliance, who will use it to achieve the first of a series of victories that ultimately bring about the end of the Galactic Empire. He stops fighting. He holds his lightsaber nobly in front of himself, and allows Vader to strike the killing blow. "You may strike me down, but I shall become more powerful than you can possibly imagine."
Except it never happened. Why would Kenobi put himself in such a position? So pointless, considering the Imperial forces actually allow the Rebels to escape, an entire orchestrated but toothless pursuit through the halls of the space station leading to such a dramatic display of heroic sacrifice?
Here's how things really went:
After the Millennium Falcon is brought aboard the Death Star and Han Solo helps everyone aboard evade immediate detection, Kenobi makes the decision to deactivate the troublesome tractor beam himself while Solo and Skywalker rescue the captive Princess Leia. That's what he says he's going to do, but it's completely unnecessary. Elsewhere, Vader and Governor Tarkin have already instructed their troops to ignore the interlopers. They're insignificant, falling into a trap. What Kenobi instead does is leave the station in a different ship, a TIE fighter he hijacks as a survey mission departs to examine the remnants of Alderaan.
He has no reason to confront Vader. A long time ago, they fought each other already, and then the Empire won the war and he was the last of the Jedi, along with Yoda, and they both went into exile. Neither of them could defeat the Sith. That was how Vader came into existence, in the face of failure. Kenobi had won that fight and they still lost. All that was left for him to do was watch over the son of Vader, Luke Skywalker, ensure that the Emperor's obvious plan of fulfilling the prophecy of bringing balance to the Force would play out, whether it meant Vader or his son ending the conflict between the Jedi and the Sith once and for all. Finally, only one side would be left standing. And it would be the Sith. Father would kill son, son kill the father. It didn't matter. Only the Emperor, also known as Palpatine, also known as Darth Sidious, would survive.
He would forever consider himself a coward, but Kenobi didn't have to die in a pointless duel with Vader for any of that to happen. He believes Skywalker will prove the Emperor wrong. He will make sure that Yoda finally steps in and train Skywalker. Kenobi's mentor Qui-Gon Jinn had learned how to project himself after death by way of the Force back among the living. Kenobi could have done so himself, but what would have been the point?
No, he's there to watch things play out. He comes back. First he helps Skywalker and Solo blow up the station, betrays Vader as they pursue Skywalker down the trench. Then he finally comes clean and tells Skywalker everything. He makes Yoda do the same. All their sins revealed. That was what the Jedi needed to finally defeat the Sith. Cleanse the darkness. Yoda dies of old age. Kenobi lives on, watches as someone else becomes the hero he could never be. His sacrifice is to live, not die. In time, his name is completely forgotten. It's happened to him before. He retires, again. An old man in the desert. Perhaps, one day, he'll be needed again...