Obi-Wan DIED In Revenge Of The Sith (From A Certain Point Of View)
From a certain point of view, Obi-Wan Kenobi died alongside Anakin Skywalker during their fateful duel on Mustafar in Star Wars: Episode III - Revenge of the Sith. Some of the biggest retcons and mysteries throughout the entire Star Wars saga can easily be hand-waved away by saying it came "from a certain point of view". That line was infamously used by Obi-Wan in Return of the Jedi when Luke Skywalker confronted him about Darth Vader and the truth of what really happened to his father.
Of course, George Lucas didn't know he'd retcon Darth Vader to be Luke's father when he originally had Obi-Wan tell Luke in A New Hope that his father was murdered. But it made sense in the end; Anakin Skywalker the Jedi died when he turned to the dark side and became Darth Sidious' apprentice; he helped enact Order 66, killed younglings, and moved to destroy the Jedi. So a part of him was murdered that day when he sided with the Emperor, and then another part of him was destroyed when he truly became Vader - after fighting his Master and being dismembered and scorched on the surface of Mustafar.
While that was the day Anakin died and Vader was born, it's possible that Obi-Wan died as well. The moment he was forced to cut down Anakin, he not only lost his apprentice but his friend too. Obi-Wan loved Anakin as a brother despite the Jedi teaching their disciples to refrain from attachment - and Obi-Wan's identity was shattered when he left Anakin for dead. It would explain why, at some point after Revenge of the Sith ended, Obi-Wan became Ben Kenobi; he failed his apprentice.
Even when Luke suggests to his uncle Owen in A New Hope that Obi-Wan might come looking for R2-D2, Owen Lars responds, "He won't. I don't think he exists anymore. He died at about the same time as your father." Although they didn't show it to Luke, Owen and Beru Lars knew who Obi-Wan really was, possibly what happened to Anakin, and what it all meant. Again, from a certain point of view, Obi-Wan the Jedi Master died when he defeated Anakin and left for Tatooine. And it wouldn't be the first time that a Jedi or Sith has shed their identity for a new one either.
Anakin becoming Vader, Count Dooku becoming Darth Tyranus, and even Darth Maul becoming just Maul all follow a path in Star Wars upon which one life ends and another begins - not just a simple name change. Obi-Wan's "death" may have been more symbolic than physical, seeing as Anakin's eyes turned yellow when he turned to the dark side and Obi-Wan remained the same (even choosing to keep his Jedi robes), but the identity shift could've had a more profound meaning in the long run than many people may have realized.