Yoda and Obi Wan split up to go kill the two Sith. Obi Wan battles Anakin, defeats him and believes that Anakin is dead. One Sith gone. Yoda goes toe to toe with Darth Sidieous and eventually backs down to run away. He says he is going to go into exile. Returns to the Tantive where Obi Wan eventually shows up. Now, the belief between them HAS to be that Vader is dead. Sure, in the 19 years in between, they'll realize that Vader was NOT in fact dead, but at that moment, we have to assume that Obi Wan believes he was successful.
So, with two powerful, healthy Jedi...why did they not go confront the Emperor and end the Empire in it infancy? Surely Obi Wan and Yoda had to assume they would have a better chance of killing Sidious than Anakin's offspring. The twins were kept in secret from the Emperor at the end of III not from Vader. Their focus should have been on finishing the job, not waiting 20 years so they could be old and less powerful, but have two other younger Jedi. Yoda and Obi Wan were as strong with the Force as they were ever going to be...so why not do what was neccessary, instead of putting all the pressure on Luke?
I wonder if Lucas even realized that, or just thought it wrapped up neatly for ANH? I mean, I'm not a Lucas basher, but that is a pretty big leap.