I don’t know exactly what those different ways it would be unfair are in total, but I would assume that some of that has to do with Joss Whedon, who isn’t exactly some Joe Schmo director Warner Bros. plucked from obscurity to finish the product. He’s a heavy hitter in his own right, and assumedly had his own ideas for how the project needed to be finished. Once people–in this cast the cast and crew–get into a groove with working with a new person, presumably it would be difficult for the original director to come back in any way, shape or form. No one wants to step on anyone’s toes. Zack Snyder stepped down, and now he feels the best thing is to step out of the way and let them get the job done.