He's done as much as Shanahan did as a coordinator in San Fransisco, or as Belichick did w/ the Giants, Jets, and Patriots before taking over as head coach. Those guys failed in their first attempts, got on the staffs of experienced coaches, and then performed well enough as coordinators to earn another shot as head coach, where they did better. I'm not saying Gregg will have the same level of success, but I'm also not saying it's a fact that he won't. Why? Because I don't know that to be a fact.
Neither do you.