Obi-<PERSON> is not there to defeat <PERSON>, he is there to defeat the <PERSON> aspect of <PERSON>'s personality.
Consider that <PERSON> goal here is to goad <PERSON> into defeating him instead of coming in contact with <PERSON> before <PERSON> has obtained more Jedi training from <PERSON>.
And what better way to put your opponent into the mindset that they are in control of the situation than by pretending to be afraid, or making wild claims about becoming more powerful? Or cheekily referring to him by only using part of his name/title?
<PERSON>-<PERSON> needs to get <PERSON> off balance enough for <PERSON> to attack him without thinking and defeat him - as a distraction for <PERSON> to get away. Can you imagine what would have happened if <PERSON> had time to think about the confrontation, and captured <PERSON><PERSON><PERSON> instead? <PERSON> could have used <PERSON>-<PERSON> as bait to reel <PERSON> back in. <PERSON> must of foreseen this and decided to take this seemingly futile path (but which led to eventual success).
By referring to <PERSON> as "<PERSON>", and feebly claiming that he (<PERSON>) can't win, <PERSON> places himself in a position as a-soon-to-be-be-defeated adversary. 605
Perhaps <PERSON> was bluffing about becoming "more powerful that you (<PERSON>) can ever imagine" (after all, all he becomes is a voice in <PERSON>'s head - right?)
But it's not until later that the audience and <PERSON> realize that <PERSON> wasn't talking about their confrontation (<PERSON> <PERSON> final battle) when he was referring to Darth's (<PERSON>'s) ultimate lack of "win".
<PERSON>-<PERSON>'s "win" is the ultimate reconciliation of <PERSON> and his son, <PERSON>. Which has the side effect of re-balancing the force, (or do I have that the other way around?)
<PERSON> has to be taken out, so <PERSON> sacrifices himself so that the confrontation between <PERSON> and his father is delayed until later, after <PERSON> has a more complete perspective of their actual relationship. <PERSON> isn't taken out by the lightsaber battle - he seemingly wins.
But the way <PERSON> disappears upon his defeat plants a seed of doubt in <PERSON> side of <PERSON>/Vader, which eventually grows to the point that <PERSON> is able to resist the <PERSON> persona and confront the emperor at the crucial time. 605