Garrett inherited that offense - he didn't build it. Big difference. It was Payton & Sparano who were the architect of that offense not Garrett.
Garrett just went along for the ride soaking the undeserved compliments while waiting for Uncle Jerry to give him the keys to the head coaching gig.
The system degraded overtime because Garrett slowly started to implement his own style into the mix, taking away the effectiveness of the talent the players had and making it a scheme-based offense like you see today.
I'm assuming that Jerry bought into the Red Headed Genius misconception that Garrett was really that - a "genius." Thus he went out and got got Roy Williams. A receiver they thought Garrett can work with and build a great offense around. That turned out to be a disaster because Garrett doesn't know how to use the talent given to him. Instead he just runs the same vanilla plays of the 90s and expects players to "execute."
Same issues are happening to Bryant right now. Garrett has no idea on how to draw up a game plan utilizing his talent. You don't see Bryant going in motion or in the slot or running slants or rub routes. Thats not part of Garrett's offense. He uses Bryant as a glorified decoy and run him deep while Witten and Beasley comes underneath on shorter routes.
There has been very little variations on this offense back then and not much has really changed till now. Thus we all wonder how we have all this talent and yet we are not able to produce anything. Its not the players. Its what you call the Garrett playbook and its holding this team back from realizing their potential.
In my opinion, this is the biggest obstacle why our offense has sputtered throughout the years. It only started becoming efficient when Romo is allowed to change the plays his way and allowed to make changes at the LOS pre-snap. But Tony can only do so much.
The biggest lost to this team was Callahan. Not because he was a great playcaller or the best oline coach in the NFL has ever seen. It was because Jerry gave him some of the playcalling duties and he inserted some of the much needed changes to this offense. Unfortunately, he was driven out by the FO. Him and Garrett did not see eye to eye. Thus we are now back to the same ole' vanilla offense - a curse to this team and to this organization.