It's more about having the right players in the right scheme. You need both. The player must be talented enough to produce when put in the scheme best suited for that player's role.
Let's look at two recent players, both in the HoF, who didn't play for the Cowboys: Ed Reed and Brian Dawkins.
Ed Reed's first four years in Baltimore had him playing under defensive coordinator Mike Nolan. Nolan had him playing SS about 65% of the time. Reed produced 22 int's in those four years, as Nolan recognized Reed's talents and adjusted him position/scheme from game to game when needed to get the best out of his player. Nolan saw that Reed could play FS and SS in his system, and didn't try to force him into a role he was not suited for. Rex Ryan took over for the next four years, bringing his system in. Reed produced 24 int's in those four years, as Ryan moved him exclusively to FS for his system, recognizing that he had in Reed the perfect player to man the FS slot. Having great LB's up front helped, of course. Ed Reed finished his career with 64 int's, 11 forced fumbles, and 646 tackles, of which 534 were solo.
Brian Dawkins played CB and FS in college but was exclusively a FS in the NFL. His first three years showed that he was a good talent, but no one really knew how good until 1999 (Pro Bowl #1 for Dawkins), when Andy Reid was hired as the HC of the Eagles. Reid did something that changed Dawkins' career forever: he hired Jim Johnson as his defensive coordinator. Johnson, who had more effective blitz packages than anyone I ever saw, needed a FS that could not only cover, but be a vicious attacker and sure tackler all over the field, one who could stand up at the LoS if necessary and handle business. He saw in Dawkins that player, and became a key cog in Johnson's defensive schemes. Dawkins played most of his career under JJ: Sean McDermott took over when Johnson passed. Dawkins career int total of 19 doesn't seem impressive, but that was not the main part of his role. Dawkins forced 36 fumbles in his career, more that Polamalu, Reed, and Roy Williams combined. He had 1147 tackles, 911 of them solo, again, a reflection of his role in JJ's and McDermott's defense. He was 4x's first team all-Pro under Johnson. For the record, McDermott, Rivera, and Harbaugh were all position coaches under Johnson.
The key here? A smart defensive coach finding or identifying the right talented player to fit a role in the scheme.
Now, do we here in Dallas have anyone coaching the defense that can scheme and identify player strengths and weaknesses the way, say a Jim Johnson, Nolan, Rivera, or Harbaugh can? We won't have a great defense unless we find a coach who can, a head coach who will leave him alone (Reid was smart enough to just leave Johnson alone), and an owner that will just get the heck out of the way for football people can run a football team.