The Cowboys played the situation right, they just drafted the wrong DE.
TJ Watt's injuries were a legit concern, but he was an awesome prospect. He had huge production in his junior season, and just smashed every measurables box you want to see in a pass rusher (1.59 10 yard split vs ~1.60 target, (10'8" broad jump vs high 9s target, 6.79 3-cone vs 7.20 target). His metrics showed that he was a speed-rushing machine who was going to be really productive if he could stay healthy.
Taco was a better prospect than pro, but he shouldn't have been a 1st round prospect to start with. His volume stat production was very good, right in line with Watt, and his pass rush win rate was middle of the road. But his measurables weren't there. He had a 4.92 40 and 1.69 10 yard split, which is just not going to get it done for NFL speed rushing. His 9'6" broad and 33" vert were low for a DE. He did have a 7.17 3-cone, which is good for a big man.
All that says that Taco had some game, but he wasn't fast enough or explosive enough to make it as an NFL edge rusher. In 2000, Bill Parcells might have drafted him to play 5 tech, but in 2017, he was a DE/DT tweener. Kind of like a taller Tyrone Crawford.
Marinelli stuck him at right end, asked him to speed rush, and he flopped.