I’m still trying to make my mind up about Obi-Wan letting Vader live; I do think it was an intentional thematic decision, because it dovetails with Obi-Wan’s remark to the escapees that “You’re what needs to survive now” and his reassuring Reva that our choices in the here and now, and not the past, are what define us. My interpretation was - Obi-Wan is done with the old fight, and he’s no longer going to allow himself to be haunted by Anakin; instead, he has finally embraced Qui-Gon’s teaching of the “living Force”. At the end of his arc, Obi-Wan has become altogether more passive; accepting; fatalistic - which coheres with the character’s actions in ANH, when he chooses to sacrifice himself as opposed to fight Vader tooth and nail.

But!..

If an edit was to be attempted that gave Obi-Wan a more explicit reason NOT to kill Vader; could Obi-Wan sensing Luke in danger not be used? Bring Obi-Wan’s visions of Luke’s peril further forward, so that he senses this soon after Vader is defeated and so rushes away more immediately after their duel? It would probably entail losing the “So long, Darth” line - which is shame!