If you look at Lucas' work - from American Grafitti through to Empire, you can see the maturity and quality - everything from Jedi onward - after he fired Kurtz - clearly has a more uniform look and feel, and it's a poor one. It seems clear that Jedi is the first film that is the pure creative expression Lucas, doing everything he wanted exactly the way he wanted - no collaboration, no input from anyone, and being surrounded by "yes men."
Gary and probably his ex-wife Marcia Lucas had far more input and say than any of us will ever know.
I really wish Kurtz would do a podcast commentary for Star Wars and Empire so we can hear who really contributed what.
A girl I know who went to school with Kurtz's daughter told me how she would talk about how upset her father was after George fired him. A quote from her attributed to Lucas was that (in refernce to Empire) "He and Kershner stole my movie from me."
Here's what I think happened: It's well documented that, after finishing Star Wars, Lucas was miserable and (according to a friend who was in contact with Lucas duing the making of SW) even suicidal. He hated directing. George's definition of directing was "coming to work with 100 problems every day and being able to solve 50 of them." He didn't enjoy it and swore he would never direct again.
He was so disenchanted with making SW that he PURPOSELY took a back seat and let Kurtz, Kersh, Leigh Brackett and Kasdan do most of the work. But, part way into the film, once Lucas had physically and emotionally recovered from SW, the train that was making Empire was moving full steam ahead and he realized it was mostly moving along without him. Everyone knows Lucas is not a very forceful guy; shy and quiet has always been his trademark. He probably didn't have it in him to step up and tell everyone to take a hike and do things HIS way - so he quietly steamed about it, did what he could, and when it came to Jedi, he fired everyone he felt threatened by (his wife and his producer) and from then on just hired people who would take orders.
There are MANY recountings of Lucas on set for ROTJ basically countermanding what Marquand was telling people - Lucas practicaly directed the film behind Marquand's back, but unfortunately Mr. Marquand isn't around to tell us more about it.
Anyway, this is my take on things, and I think it's obvious because of the downward spiral that started with Jedi and has remained the status quo for Lucas ever since.
I'm sure Kurtz doesn't want to sound like sour grapes and start saying "this was my idea, that was my idea" but I think fans would really like to hear more about the process of making SW and Empire and get a better idea of how it went wrong from there to Jedi.