I hope you're being sarcastic. When is the last time a coordinator in the NFL was promoted to head coach after the head coach got fired?
Off the top of my head, Mike Singletary (yeah, I know how that worked out but it's an answer). The bar is so low in Jax that he can't possibly be blamed if they mail in another 4 win season. The company line will be that the mess was there before he arrived. But he'll get some credit if they become good by some miracle... Is it a great move for him? Definitely not. But it's something.