Oh don’t get me wrong, I also feel like just having Obi-Wan walk away is a weird choice that I don’t agree with. Especially when Obi-Wan, earlier in the same episode, said, “It ends today.”
Just to have him walk away again, it basically puts us back at the end of ROTS, except Obi-Wan is less guilty now I guess.
And that alternative isn’t bad, but you still end up with Obi-Wan leaving Vader for dead, and not actually making sure he is dead.
I say it’s bad only because it makes Obi-Wan look immensely irresponsible for the sake of keeping the status quo (not that continuity errors seem to bother these writers anyway but killing Vader is a pretty big one). We know the circumstances around why he left Anakin in RotS and it’s much more understandable why he couldn’t bring himself to do it then. It wouldn’t have even seemed likely Anakin could have survived. Now we know that not only did he, but he’s been killing people, Jedi and otherwise, for a decade.
I wouldn’t have as much of a problem with it if it didn’t fundamentally damage Obi-Wan’s character. Worse, it makes everything Luke goes through in the OT feel even more pointless.