can we start a Nagy subforum?
A. Because he has made many mistakes with play calling and has lapses in managing a game tempo
B. His players seem pretty unprepared at times, specifically on the offensive side of the ball
C. He hasn't improved the offense even tho a ton of money and several new players have been brought in.
So what you're saying is that he isn't a perfect coach, and he isn't better than every other coach to ever coach an NFL franchise.
Both Pace and Nagy have had some startup issues. Pace's first ever draft pick was Kevin White. Every pick since then has been better than that pick (not hard, admittedly). Trubisky was a miss, but most of the pundits out there had Trubisky as their #1 or #2 quarterback, and none of them had Mahomes rated over Trubisky. So Pace looked at those results, saw that the Chiefs had a better 'process' than the Bears did, and went and tapped into that 'process', bringing Nagy to Chicago. This year's draft shows SUBSTANTIAL improvement over previous years, and in previous years, we got some misses, but also players like Eddie Jackson, Khalil Mack, Roquan Smith, A-Rob, Cody Whitehair, James Daniels, David Montgomery, Darnell Mooney, and probably some others I've forgotten. Now we add Justin Fields, Teven Jenkins, and hopefully a few of the late round picks this year show some promise.
To address your objections point by point.
A. Sure, there have been errors in play calling and managing the game. Name a coach who hasn't had those. Matt LaFleur kicked the field goal at the end of the NFC championship game last year, clearly a bad decision. Should GB fire him for that? Clock mismanagement happens all the time. As a fan, we don't like it, but to expect perfection from a coach is asking too much. You'll end up firing a bunch of really good coaches because you want a perfect coach.
B. Again, that happens, and over time, as he works with Pace, the players they have will be better at managing that stuff. Another example - Bears beat the Bucs last season. It was early, and the Bucs were still figuring things out. Brady was driving at the end of the game, but he lost count of the down, throwing an incomplete pass on fourth down. He signaled the ref that it was third down, and the ref pointed to the marker showing it was fourth. So Brady couldn't keep track of the down, and Arians, his coach, should have made sure he was prepared to do that basic function of knowing what down it was. Should they have fired Brady and Arians for that gaffe?
C. The problem with the offense was that Trubisky needed more time, more development, or more skill to execute it. It's as if Nagy went into a boxing match with one hand tied behind his back. You can't judge the ability of a play caller without giving him players that can fit into his system and execute his vision. Bill Walsh coaching the horrible Bucs teams of the 1970's would have had limited success at best. Bill Belichick would not have been able to do much with the Lions in 2008, although he might have gotten them from 0-16 to 2-14.
I get the impression that you expect a head coach to have a godlike ability to transform players into all-pros and to make every call correctly. I would suggest that you consider rating head coaches relative to their peer group, and not to some idealized standard that no human can meet. There are no perfect coaches, just humans doing their best.