What evidence is there that DQ wants to be a HC? Before he signed his most recent contract, he was a red-hot commodity, and had the perfect opportunity to leave. But he didn't.
Maybe...just maybe...Quinn actually likes living in the Dallas area, being regarded league-wide as a top-tier DC, having great players to work with...all without having to occupy the hot-seat & deal with all the administrative nonsense that a HC does in a completely different job.
Football coaches weigh "quality of life" issues, too. In most cases, a leap from coordinator to HC brings with it a significant financial reward. I have no idea what DQ earns, but sense that Jerry may have shrewdly removed that consideration from the table with the latest contract.