Well, Brackett didn't "remain" because she died. Apparently the treatment that she turned in was not up to scratch but out of respect her name remained on the credits. It was never on the cards for Kersh to return for the next one. And I really think that anyone who was hoping for episode six to be TESB part two is entirely missing the point of the arc of the three-act play.

What is so great about TESB is that any lesser writer would have followed up Star Wars with simply more of the same, based on the premise that if it ain't broke etc. Certainly, if Lucas had not been canny enough to retain sequel rights then Hollywood would undoubtedly have churned out a repeat of the first film.