Finally, firing someone only works if there's a viable replacement.
This is faulty logic. If you have someone failing at the role you hired them for you get rid of them, period. You don't allow the person to keep failing at their job, especially if they have demonstrated over a period of time they are not likely to ever improve.
There is always someone else qualified. Sometimes you may not know who the most qualified for the job is until you give them a chance but you should never resign yourself to being stuck with the person who isn't getting the job done out of fear their replacement might be worse.
I mean, let's pretend for a second Brad actually is he good at his job, how would anyone have ever known this previously, given he had never been an NBA coach before? He had to be given a chance to truly know.
Respectfully, you're passing some hypotheses off as fact here. Another way to look at that is, will making a change lead to a net gain for the team? If not immediately, then over a certain period of time? I would suggest you would only fire someone if either of those two conditions are met. Otherwise, for what purpose are you getting rid of them? Sometimes teams change a coach because they want to go in a new direction, or because they think the coach has gotten as far as he has. But the trigger is usually that you're unhappy with the status quo and want to improve it. But just as you don't want to be stuck with someone because you're afraid to make a change, neither should you make a change for the sake of making a change.
You actually mentioned the main issue at hand, the hypothesis that he's failing at his job. A lot of people on this thread obviously disagree with me, maybe they have a much narrower definition of failure than I am (e.g. championship or bust), but I'm not sure that's an accurate way to describe the situation we are in, in the ECF, 2-1 down to a team that eliminated the No.1 seed in 5 games. Neither is it an accurate way to describe his overall tenure here.
There's obviously objective criteria for this, you can compare metrics such as wins, points scored, defensive efficiency, etc. If we are firing Brad for failing at his job, then we would want his replacement to improve on those metrics. He's got a pretty decent resume. He's been in the playoffs every year since his first year, and he's been in the ECF 3 of the last 4 years, with 3 different teams. I'm not sure any of the names I mentioned as alternatives, unless there are some hiding in Europe, have been able to do anything similar. The only one that really stands out is D'Antoni who had 28 playoff victories with the Rockets in his time as coach, only exceeded by Steve Kerr (43), Brad Stevens (33) and a combination of Duane Casey and Nick Nurse (31). If you review the team's year on year metrics, other than last year, the Celtics have been right up there.
Then there's all the subjective criteria...you have to evaluate fit, coaching philosophy, system, all those intangible things. I'm sure those things, along with his record of taking Butler to 2 NCAA title games, played a role in getting him the job without NBA experience. I won't get into those as everyone has a different preference for a coach. Sure if you combed the NBA and the Euroleagues and college basketball you "might" find someone who could do better, but the bar would be set pretty high for them. Say if Steve Kerr or Pop became available? Or Spoelstra? Those are guys who could probably meet that bar. Maybe Jay Williams at Villanova?
But from what I can see on the 10 pages of this thread, as scottiej said, most of the complaints seem to be due to his personality. People seem to want a fire and brimstone coach who is much more autocratic than Brad is, who will call timeout instead of letting the players play, who will yell at them if they mess up, and criticize them in the press when they make mistakes. It's a personality thing. Because on his body of work it's pretty hard to criticize, in my opinion.
But hey, as long as the team wins, whether it's in spite of Brad or because of him, it's all I really want. I will keep voicing my opinion about Brad's shortcomings until lit is no longer an issue no matter how much it irritates some of you, however.
We're all entitled to voice our opinions, no matter how irritated others may be. So keep voicing away