I think ROTK is trying to make a different point, though. all throughout LOTR, we've seen initially good people around Frodo succumbing to the Ring through their weaknesses, while Frodo remains practically incorruptible. The fact that he doesn't come around at the end, and that the Ring is only destroyed because of external forces, drives it home that even Frodo isn't beyond the reach of evil. Luke's journey, on the other hand, is framed as a question of whether or not he can avoid going down the same path as his father. The payoff here is that he isn't doomed to repeat Anakin's mistakes, So the satisfying triumphant ending fits. I think both endings work wonderfully for their respective works, but they're almost negative images of each other thematically.