Your argument doesn't make any sense. Every team gets a first round pick every year. The Cowboys have developed all those players since he got here, he didn't inherit any of those players. Dez was a hot mess for good chunks of his first 2 years here and many people called for his release multiple times on this site and in the media - he became the player he is today under Garrett's direction and you can't assume that would have happened anywhere else. Many teams have missed on picking OL that many people here coveted (Eric Fisher, Lane Johnson, Jon Cooper, Chance Warmack, Justin Pugh). Heck, the Travis Frederick pick was ripped to shreds here. If drafting OL were such a blueprint to success how come the Cowboys hadn't done so in about 30 years before Garrett arrived and yet 3 out of 4 years we went that way in the draft during his stay here and hit a HR each time.
I don't believe we had the talent to be a definitive playoff team but we did compete for a playoff spot every year of his tenure here. That is a good coaching job by almost any standard. Our starting interior OL back in 2011 was Bill Nagy, Phil Costa and Kyle Kosier playing on his last legs.
The part that he did inherit was a top 10 QB, a great TE, a great WR who couldn't stay healthy (Austin) and 1.5 seasons of a great pass rusher (Ware). There was talent but outside of the QB position a bottom 10 roster in the league overall the day he took over.
Go back and look at the 2010 roster he inherited and explain to me where all the established talent to build a playoff team was outside of Romo, Witten and Ware. And remember, we are talking about their futures, not their past. I've already made my case there wasn't much established talent to work with but show me what I have missed or discounted. Since those players are no longer here justify the strength of that roster by showing they were great players by being successful elsewhere after they left.
Here are the guys I can think of. Marty Bennett who was a knucklehead and didn't want to backup Witten anymore. Terence Newman who had a very bad second half of 2011 before rebounding with the Bengals as a 3rd CB. Demarcus Ware who is a nickel pass rusher who has gotten about 5 sacks in the second half of the last 3 seasons. Jason Hatcher who at 32 years old was not part of our future is a solid DT with the Commanders but did play at a high level for us from 2011-2013. However, in Hatcher's case he got no offers in UFA in 2011 before coming back to the Cowboys and only established himself thereafter (i.e. he wasn't really an established player back in 2010).