Some players are just bad fits for certain types of offenses and defense.
Remember when Parcells came in and they eventually switched to the 3-4 defense? With a 3-man D-line, the LB's must be big and quick. They don't necessarily have to possess a high football I.Q. They must be instinctive.
At the time, Zimmer was the Cowboys DC. His prior defense, a 4-3, featured two undersized, but effective LB's, Dexter Coakley and Dat Nguyen. They were just not good fits for the 3-4.
In addition, Greg Ellis, a prototypical DE in the 4-3, was switched to a 3-4 OLB late in his career.
Therefore, despite the fact that the Cowboys defense was #1 ranked while still using a 4-3 defense in 2003, Parcells made the switch and basically drafted his 3-4 in 2005.
It's possible Fehoko and Scott could still end up having great careers within a different system, it happens.
Under a completely different system, former Pro Bowl FB Daryl Johnston might have been evaluated based on his running ability. He might not have made it through his first training camp.
In the more sophisticated offenses of the late 80's, most O-lines required less bulkier, quicker guards. Nate Newton can thank Jimmy's offensive style for launching his 6-time Pro Bowl, 2-time All-Pro career