At the end of Revenge of the Sith, Obi Wan and Yoda went into hiding. They thought they were not able to defeat the Sith and the Empire. They hoped that the newborn twins (Luke and Leia) will be able to someday.
Yoda was nearly a match for the emperor in Revenge of the Sith. In Return of the Jedi he was old and died.
Obi Wan beats Anakin in Revenge of the Sith and (as far as I know) Anakin was stronger than Darth Vader. So Obi Wan could surely beat Darth Vader shortly after Revenge of the Sith. In A New Hope Obi Wan was old and not able to beat Vader and died.
Luke (even after Yoda’s training) was not nearly able to oppose Vader (in The Empire Strikes Back). I do not know what happened between The Empire Strikes Back and Return of the Jedi that makes him better, but even in Return of the Jedi he barely beats Vader. He wasn't remotely a match for the Emperor, who was only killed due to the surprise attack from Vader.
The Empire was just born at the end of Revenge of the Sith. In Return of the Jedi it was consolidated and had stabilized over 20 years.
So my question is, weren't the chances of Obi Wan and Yoda at the end (or shortly after) Revenge of the Sith to take out the Emperor, Darth Vader and the Empire much, much greater than the chances of Luke when he (indirectly) finally did it in Return of the Jedi? We can even discuss if Luke contributed anything in taking out the Empire, the Emperor and Vader, because he wasn’t involved in the destruction of the Death Star II (over Endor)—that would have happened even without him—and would have killed Vader and the Emperor also.
So was this a great misjudgment from Obi Wan and Yoda that the baby twins, or at least one of them, years later will be a better match than themselves?
Bonus Question: If the sake of the whole galaxy is at stake and Obi Wan and Yoda became old and weak, why didn’t they start to train Luke earlier? Just because uncle Owen didn't like it? Or did they resign and give up over time and the coincidences in A New Hope just woke them up again?