And Lucas and Kurtz split for the reasons Cassidy listed above. It wasn't altogether acrimonious, and it wasn't a total split, either--Lucas ended up finishing the financing for and helping organize "Return to Oz" when Kurtz went overbudget and way late with THAT film, as well. If I remember right, Kurtz had a breakdown on set in the last month, and physically couldn't do his job. That's when Lucas stepped in to help Kurtz. Didn't have to, but did it as a favor to Kurtz for all the work he'd done on Star Wars and Empire. Kurtz soon had to file for bankruptcy afterwards.
It wasn't at all that Kurtz saw "Star Wars" going down the tubes, and it wasn't that Lucas got a big head and wanted things done his way all of a sudden. Lucas ALWAYS wanted things done his way. His relationship with Dykstra (and the British crew) on Star Wars was strained because Lucas was acting as Cameron is known for acting--getting down in the muck and doing the crew's work FOR THEM if it wasn't just as he liked it. The heads of the departments didn't appreciate that, obviously
But it does prove that Lucas was always very adamant about getting what he wanted when he could control it. And if he couldn't control it, he got frustrated. And Kazanjian just left to fly solo. Lots of people did it. Hell, Joe Johnston, Frank Darabont and David Fincher all left Lucasfilm/ILM to do their own thing. It's not always some acromonious behind-the-scenes drama.
And McCallum isn't exactly a no-talent. He's very good with the money, keeps costs down, keeps the production moving fast, and after reading "The Making of Revenge of the Sith," the diaries show he's not at all the "yes man" people constantly cast him as. For the majority of the book, he's prodding Lucas to either a) get to work on the script or b) stop tinkering with the film unnecessarily.