That's one more in the list Why I don't like the prequels. During Anakin's "training" I would have expected to see some of Kenobi's demons, the ones that made him fail Anakin. After all from Ben's POV he always said that he had failed him. I think that deep down Ben knew that Vader couldn't be defeated by force, actually he never says so. IIRC, he only insists that Luke has to confront him again (after all that was the point of saving, and protecting the twins 20 years earlier). In terms of power, I don't think that Luke could have defeated Vader, the only way to kill Vader was through the Redemption of Anakin Skywalker. In ROTJ when Luke talked to Kenobi's ghost, Luke insisted that he needed to redeem him, that in his core Vader wanted to be redeemed (I pick this from I, Jedi, where Corran interprets that Vader's admission to Luke was in fact a silent bid for help). Obi-Wan insisted that Vader couldn't be redeemed that he was more a machine than a man, that there was nothing in him left to be redeemed. Couldn't Obi-Wan have said this only to make Luke push harder and redeem him in the end? It's all a matter of interpretation, though.