Hill hasn't proved anything in Provo YET. (emphasis on yet) In points allowed BYU ranked 99th last season. I get it that it will take a while for Hill to get his guys in and the defense had a lot of new faces last year, but it's not like he's become indispensable. I'm optimistic that he'll be a good defensive coordinator, but you have to also realize that that hasn't been proven on the field.
If he goes to USU, it's just an opportunity to prove himself as a head coach.
Coaching at USU won't stop BYU from hiring him as head coach if BYU moves on from Kalani.