"You can't win - but there are alternatives to fighting."

That line pretty much sums up what Obi-Wan did on board the Death Star. While he did technically "fight" his real purpose was not to fight and achieve victory, but to use that fight to win on another level.

The simple observation is that Obi-Wan served as a distraction to enable Luke's escape, and that when he died he would gain new power. With his new power he would become one with the Force yet still be able to mentor young Luke and provide him guidance.

Another simple observation would be that Obi-Wan knew that Vader's powers exceeded his own; the Dark Lord was no mere acolyte any longer. Yet, considering the meditation and tutelage Obi-Wan gained over the years on Tatooine, combined with what appears to be little or no slowdown physically, one could say that Obi-Wan was an even more powerful Jedi than when he first faced Vader some twenty years past.

Obi-Wan had become a true Light Side Master.

Yet Vader, for all his power, was still playing second fiddle to Emperor Palpatine, the true Dark Side Master who would never fully divulge the extent of his mastery to Vader, lest he fall victim to same mistake that Darth Plagueis committed.

Vader claimed to be "the Master" yet true scholars of Sith ways know this to be false. Under the rule of two, there is always one to embody power and another to crave it. Vader was dependent on his Master who would feed him Sith training here and there like a pet. So Vader was not a true Master of his side of the Force, while Obi-Wan was.

So with that explained we clearly see that Obi-Wan was the more powerful of the two. And that means he could have beaten him.

Considering what Vader had become, Obi-Wan would certainly have no remorse in striking down his former apprentice; any connection they had was severed long ago. Yet he did not strike him down. Why?

Because Vader could still be saved, and Obi-Wan was right in that opinion now more than ever. But Obi-Wan could not save him, only Luke could.

Even beyond that though, there is a more important fact that remains: only Vader had the power to destroy the Emperor, and only Luke could give him that power.

Obi-Wan could have beaten Vader but in doing so he would have condemned the galaxy to Palpatine's rule and the Jedi would truly be lost. He knew there was another way, the only way.

Obi-Wan's time had come and gone, and now the time for Luke was at hand. In realizing his destiny, Obi-Wan enabled Luke to fulfill his destiny which in turn enabled Vader to fulfill his destiny, the one he had been chosen for all along: bring balance to the Force.

Despite the horrors and evil of the Emperor's rule, despite the sins of Vader, only through this difficult path could the Jedi realize their error and be reborn anew, and only through this path could anyone have defeated Palpatine.

Obi-Wan's sacrifice set off a chain reaction that saved the galaxy.