Farrell isn't a bad manager.
Farrell isn't a good manager.
If the team is going badly he won't pull wizardry out of his hat to save it.
If the team is doing well, he won't make boneheaded decisions that torpedo its success (fan wangst about small insignificant moves notwithstanding).
Because all managers take risks and play the odds, there will be times when Farrell makes moves that don't work. These will include two kinds of moves that don't work -- calculated risks that don't pay off, and moves that seemed right at the time but did not pan out that way He will also make many moves that do work, and well be ignored because they did work and so everything is fine.
Because the focus isn't (and can't be) always on winning the game right in front of you, fans can always point to times when Farrell made decisions that didn't win the game in front of him, at least in their own heads.
All of these things are part and parcel of an average manager. No one who has said words in this thread in the last 6 months has said anything new or interesting. Farrell isn't the greatest manager in the history of ever but he is also nowhere NEAR bad enough to worry about replacing. He is right in the middle, a solid rank-and-filer, and anyone you replace him with will be subject to every criticism our smallminded fury idiotically assumes is somehow uniquely his.