As mentioned before, every team has had to deal with personal decisions that didn't pan out the way they planned and hindsight makes them look like bad decisions (this gives purpose to all of the draft do-over articles written every year). And dealing with the salary cap is a problem for every team, especially when your players have individual success (see below)
From my perspective, it isn't poor personnel decisions that has held this team back, its lack of coaching and poor schemes. Garrett has arguably had one of the most talented rosters of any head coach during his tenure. He had the most drafted pro-bowlers, including over 20 pro-bowlers (one at every position accept safety), 9 first team all-pro players, 4 future HOF players, (Witten, Ware, Martin, Smith), 1 who is at least worthy of HOF discussion (Romo), and 2 more with potential depending on how the rest of the careers turn out (T-beard, Zeke). He has had 2 QBs that were/are top 5-10 statistically in almost every category when compared to their peers during their tenure (getting both for only 1 4th round compensatory pick). All of this, while picking outside of the top 15 in 11 of the last 15 drafts (highest picks for Garrett - 4 (Zeke) 6 (Claiborne) 9 (T Smith).
All of this talent and only having 3 playoff appearances in 8 years and 2 wildcard wins is direct indictment on the coaching. iMO, all of this talk about this teams shortcomings are because of Romo, Dak, Zeke, the defense, special teams, etc., is misguided. Lack of quality coaching, antiquated schemes and lack of game planning are the biggest reasons for the Cowboys failures the last 8-10 years.
Jerry, has done a relatively great job with his personnel moves lately (by listening to his scouts) , its the fact that he needs ALL of the credit for his teams success and chose inadequate coaches that has mired this team in averageness.
I will admit, listening to Marinelli in the draft room prevented Jerry from having better personal success on draft day.