My short answer to this is I'd say the vast majority of the "blunders" discussed here week in and week out during the season aren't actually blunders but are fans second guessing tough decisions that didn't work. There are definitely examples of bad management, but those are overblown too, and for the most part were things that happened earlier in Garrett's tenure.
I'm not sure which players have left Dallas and gone on to play better elsewhere, unless you're talking about Weeden, and that debate boiled down to nothing more than his team winning some games against bad teams when he happened to be quarterbacking and not Brandon Weeden actually playing any better.
For sure, there are a lot of criticisms leveled at Garrett. But that's because of the losses and not necessarily because of the coaching. I will say he's a pretty average game day coach (better now that he's got better coordinators). It's what he does the rest of the year where I think he earns his keep. He's a good coach in a high profile job in a market where building a winning culture and a winning roster is an extra difficult challenge.
Boy, it would be tough for anything to be as on target with my thinking as this is. That literally is about what I would have written as a response here (although probably not as eloquently as this).
Particularly the part about his game-day coaching vs what he does the rest of the year.
The team is disciplined, motivated, and plays hard as hell for him. He is improving as a game day coach but how he runs the team is as good as one could expect for anyone working under Jerry. The things that I look for to form an opinion of how well coached a football team is, are all there under Garrett. The players use his verbage daily, preach what he preaches, they have all been molded towards what Garrett wants and expects out of his players. One and all have completely bought in to Garrett's way, and it is perfectly clear that the coach provides the direction and nature of the team. They play physical, hard and to the whistle... win or lose.
His assistant coaches and coordinators love coaching for him and guys like Linehan and Marinelli... long time NFL coaches and bonafied hard arses, love the way he runs the team and agree with his methods and culture.
His first few years had more of the game day coaching mistakes as he naturally, learned along the way. His 8-8 records were none too surprising considering he needed to do a complete overhaul of the roster minus a few key cogs like Romo and Witten. I've said many times that the 8-8 2013 team was much better than the 8-8 2011 team he coached in his first full season as HC. He brought in more physical guys and he did so at positions where he thought it was important to be good at like OL.
Then, in 2014 the team started winning the way Garrett envisioned. Physical, smash mouth football with a strong OL and a running game that could impose its will. The team (and Garrett) looked like they had turned the corner. Nobody wanted to play against that running game.
So, along comes 2015 and with it came high expectations for a team that looked just about as good as any of them. The rest, we know. Romo and Dez go out and Dallas starts losing games. However, for the most part, those games were lost by a small margin at the ends of the contest. Two in overtime, more by 1, 2, 3 points... it was gut wrenching for Cowboy fans. But with a nudge in the right direction, those losses could have been some wins. Most were very close.
Then, with the team having pushed and waited for Romo to come back, he gets there and wins the Miami game before again breaking his collar bone against Carolina (which wasn't a close game BTW). The air simply whooshed out of the team. You could almost hear it.
The team was effectively done at that point. They still played hard for the most part, but by then it was clear that 2015 was a lost season and it was time for the team to sit injured guys (like Dez and Claiborne) and to play younger guys to see what they had for the future.
Garrett is as much to blame for the record as anyone... but the way things fell with he injuries and all of the close losses, it would be difficult to judge Garrett off of what went down last year.
I like a lot of what he's done and I see the improvement in the culture and philosophies that he has in place. Like I said, I saw improvement from the 8-8 2011 team to the 8-8 2013 team and then I saw things fall into place in 2014. Last season was so weird and losing Dez and Tony makes it so hard to judge.
Speaking for myself, this season should tell me a lot. I think we'll have a good idea if 2014 was just luck or the continuation of Garrett's team building. Barring catastrophic injuries to key players, we will see where Jason is taking the team this season, IMO.