I have to disagree...you can bet your bottom dollar that Jason Garrett would not leave Dallas for a HC job somewhere else just because it comes a little sooner. Face it, until Dallas stepped up, no one had even offerred JG an OC job.
He and his family are Dallas Cowboys people...I'd bet it's his dream job, and he's smart enough to wait if that's what it takes. If he's in Dallas, doing great, and JJ assures him when someone comes knocking that he's the next HC, maybe even making him assistant HC at the time, there's no way Jason leaves Dallas for Arizona or Buffalo or some other job just because he gets the position a year or two sooner.
Jason Garrett came home to Dallas for a reason...there's a reason he signed a contract without even knowing what his actual title will be this year. The only way Jason Garrett leaves Dallas is if he's unsuccessful.