Kersh wasn't pushed away, he declined to do Return of the Jedi on his own. he WAS the first choice. There was no ill will and it certainly wasn't because he worked with George. He declined for the same reasons George had Kersh direct ESB: He was tired and wasn't up to another big long shoot on a Star Wars flick. If I remember right, he was also initially thought of for one of the Prequels at one point. Kurtz wasn't "pushed away" either. The facts are pretty easily obtained via any library and any one of the three bigger biographies about Lucas: "Skywalking" which is the most accurate, although there's some contradictions from time to time, "Mythmaker" which seemed to take most of its research from "Skywalking" and Peter Biskind's book "Easy Riders, Raging Bulls" making the book feel at times like a copy of a copy, with all the inaccuracies that entails, and "Empire Building" by Garry Jenkins, which is pretty workmanlike but also gets a fair amount of details wrong.

Besides which, Kurtz and Kershner have their fair share of clunkers in their repertoire as well. there's no guarantee Jedi would have been magic once again simply because they're Kurtz and Kershner.

And Kurtz didn't LEAVE to do Dark Crystal. Kurtz was told they weren't going to use him for Jedi, due to his inability to keep the budget down and the movie on schedule, and he then went for and got the Dark Crystal job--a movie which also finished considerably overbudget and late, if I remember right.