There two parts to a baseball manager's job. First is the stuff fans can see: the in-game moves. Pinch hitting, pitching changes, double switches, fielding adjustments, etc. It's not exactly brain surgery. Any kid who's played enough baseball simulation computer games could do the job, given access to the same information available during the game that a manager has. Bobby Cox was so-so at this part of his job. Fredi Gonzalez is abysmal.
The other part of the job is the touch-feely clubhouse management stuff. It's unquantifiable, so there's no way to know how important it is. Maybe it's five percent of a manager's job; mabye it's 90 percent of a manager's job. The players themselves always say it's critically important, but then players always say they hit baseballs and make basketball shots and catch footballs because of determination and heart and grit rather than their transcendent physical skills, so they are not exactly good witnesses here. But there's certainly no question that, if a million-dollar a year manager has any value at all over a kid out of the stands, it's in this area.
By all accounts, Bobby Cox was magnificent at this facet of the game. There's no evidence to suggest that Fredi is any good at it at all, and that he might be actually horrible at it.
In any event, there certainly doesn't seem any reason not to try something else. The only way the next guy could be worse is if he went through the clubhouse with a sledgehammer, deliberately swinging at the players' knees.