Okay you want a long answer?
The Cowboys have always had an above average roster during Garretts tenure. They have had an elite O-line, a couple different elite RBs, a true top-end WR1, an (aging) HOF TE, and well above average QB play... and all of this has resulted in 2 winning seasons and a 1-2 playoff record in six years of Garretts coaching. During each of the three consecutive 8-8 seasons Garrett had near the start of his career, the Cowboys would have made the playoffs with a win in their final regular season game in all 3 of those years, and yet they lost all 3 games - which leads me to my next point.
Garrett does not win big games. What is his biggest career win? The playoff game against Detroit? I am guessing that the reason Garrett has barely any memorable wins to his name after 6 years is because -
Garrett has shown no ability to make in-game adjustments. Throughout his tenure, the Cowboys regularly perform worse in the second half than they do in the first. Here are some of the numbers from last season
1) The Cowboys scored 46 total points in the third quarter last season, while their opponents scored 100.
2) They lost four games in which they led at halftime.
3) They lost every game in which they were losing at halftime.
4) Dallas' opponents got progressively better at scoring every quarter, giving up 14 percent of their total points in the first quarter, 23 percent in the second, 30 percent in the third, and 33 percent in the fourth quarter.
On the topic of mid-game adjustments, the game last year where that no-name Falcons DE got 6 sacks is a perfect example of poor adjusting. Even in the third and fourth quarters after the guy already had a few sacks, the Cowboys barely attempted anything to slow him down (like double-teaming or chipping with a TE or whatever), if you dont believe me, just watch for yourself
So there you go, he is an underachiever who cant win big games and cant make mid-game adjustments. Do you want more reasons or is that enough for now?
Maybe Jim Harbaugh? Josh McDaniels? Bruce Aryans?
No doubt its hard to find a great coach. Theres only a small handful of them, and even when you get a great coach he still needs to be surrounded by the right situation in order to succeed.
For the record, its not like I ever said Garrett is the worst coach in the league...hes definitely better than clowns like Hue Jackson and Marvin Lewis. I said Garrett is mediocre, and if mediocre is okay with you, then by all means keep Garrett and continue to be irrelevant. Personally if I was a fan of "americas team", I would demand more than mediocre. A franchise like the Cowboys is one of the most attractive coaching jobs in the entire league, they shouldnt have to settle for mediocre.
Anyway, since I am not a Cowboys fan, I kinda hope Garrett stays another 6+ years... theres something kind of cathartic about watching the Cowboys lose.