My point was that Mace could have killed Palpatine. Palpatine did not have an apprentice at that time, Dooku was dead and Anakin had not yet turned. Had Mace been allowed to strike Palpatine down, the Sith would have been destroyed. The prophecy seemed to indicate that it was Anakin's destiny to destroy the Sith, but he already faced his destiny and walked away by that point (as you point out). It may not have been Mace's destiny to destroy the Sith, but he could have done it anyway had Anakin not interfered. And I can't imagine that had Anakin just stood there and did nothing that he would have fulfilled the prophecy since he actually wouldn't be destroying the Sith, merely allowing the Sith to be destroyed by someone else. Obi-Wan I think thought that Anakin was as good as dead. And in the case of Yoda, Palpatine had the high ground and reinforcements were likely on the way (Mas Amedda slipped out right before the fight). The odds were stacked against Yoda and after Yoda so casually walked in after Order 66 and attacked Palpatine in his own office, I don't think Palpatine was going to suffer that same mistake again. I don't think they had much of an option but to retreat. Anakin had the potential to become stronger than either Yoda or Palpatine (admitted by Palpatine himself). So Yoda and Obi-Wan were probably relying on Luke being trained into a Jedi that could achieve a similar potential and completely overpower Palpatine. Palpatine in ESB even fears what could happen if Luke becomes a Jedi, and Yoda says that it is unfortunate that Luke found out the truth regarding Vader so early in his training. They hoped that by the time Luke found that out that he would have had the proper emotional training to deal with it and still do what must be done. And really, had things gone according to Obi-Wan and Yoda's plan, I don't see why that could not have worked. Palpatine seemed to sense a future in which it was possible that the Sith would be destroyed by a Jedi who was not the Chosen One and thus was not destined to be the one to destroy the Sith.