I'm of the mindset that Arnett is a bit of a different kind of guy than many in the profession. He was coached by Rocky Long at New Mexico - Rocky was at UNM for 10 years when Arnett played for him.
Rocky Long was at SanDiego State for a decade when Arnett came and joined him on staff.
What I'm saying is that Arnett has been around, coached by, and learned from a coach who had a great grasp of being at a place and building longevity and sustained success. On the the surface, at least, it seems that Arnett has been around these guys for years and truly enjoys being around them. I CAN tell you from experience that DCs love having full reign of their side of the ball and let them do what they feel needs to be done - and it does seem that Arnett found that in Leach - a HC that trusts him fully. That makes a huge difference for a lot of coaches.
He runs an unconventional defensive scheme and philosophy than a lot of ppl at the P5 level. There's not a lot of HCs that want to risk making that switch to a 3-3-5.

UNM doesn't win much, but if you look at that state, there's not much in terms of talent to win with within an 8+ hr drive for a P5/G5 program.
having coached in New Mexico, those coaches in the HS/college ranks are phenomenal talents because they do more with less. And they are very loyal.