Why would he? That's just stupid. You could severly cripple the Empire by killing Vader, and you decide to give up your life?
Lets see. He was stronger than Yoda, he pwnd two very dangerous and wanted men in the blink of an eye, and there's no logical reason that'd he'd have declined to below Mechanical Vader's power. That's why he must have been faking.
But why would he rake? he was surrounded. What if he killed Vader? The Stormtroopers would attack him. And what if he killed those four Stormtroopers? The Death Star carries thousands of Stormtroopers, kore would arrive in no time, an alarm would be sounded, ben would never escape and Luke would never abandon Ben. So both Ben and Luke'd die.
That's why he faked.
Anyways, where in the AC does Lucas say Obi-Wan is the strongest good guy?
Right before the duel with Obi-Wan and Vader.
Also, in the ANH Novel, written by George Lucas, describes them as being equals, and describing Obi-Wan as trying to defeat Vader... So maybe Vader and Sidious are a hell of alot stronger than we thought.
The novel was written like 29 years ago. The ROTJ novel was only written 23 years ago and it says Owen and Obi-Wan are brothers. Which the movie clearly contridicts, as it does with the duel.