Kasdan did not provide for the story in Star Wars the same way he did for Raiders of the Lost Ark.
He saved that movie. Other than making the dialog punchier and tightening up the script anyone could have polished up Empire or Jedi. Kasdan never was the author. Nor was Gloria or Willard co-authors on Star Wars the way they absolutely were on American Graffiti.
Lucas is bad on dialog and bad on drama so he hires people who fix those aspects.
Lucas is 100% the author of the OT good or bad, the same way he was for the prequels. Its his story and its in no way collaborative. The stories are his and very personal. They are in his imagination and his head. No matter how much he wished he could hand it off to someone else it was impossible.
It was a mistake to give Disney an outline and to believe they wouldn’t screw it up. He has to write these things and oversee them himself to get the desired result.