I wonder if the point wasn't to train Luke to beat them (Vader and Palpatine), so much as train him enough to make him something they would finally have a showdown over. As others have pointed out, waiting to train Luke until he was older than when Anakin started would make no sense, unless they needed him to seem vulnerable due to his age and impulsiveness, but also show his potential by how much he progressed in a short time. Yoda and Obi-Wan didn't need another fully trained Jedi, they needed another potential Anakin. The Sith Lords fight over him, one takes out the other (as they've wanted to for years), and whoever's left, weakened by the fight, is ended by Luke. If that was the plan, then Leia was probably far too disciplined to convincingly be bait, which I think is shown by her handling of Vader and Tarkin on the Death Star.