If they're good, then other programs are going to try and poach them and your team will have to fight to keep them. I think it's a good problem to have (especially as opposed to the alternative). That's precisely what happened with Gary Anderson, Sitake, and Hill. Supposedly, USC made a run at Scalley over the off season and so did Notre Dame with Ludwig. Harding, the OL coach, has interviewed in the past for a NFL team or two. No insider info, but I bet that if the Utes hadn't given Scalley assurances already, he'd probably been lost already to a head coaching opening.
Whenever Scalley takes over, if Hill is still at BYU as the DC I'd bet they at least interview him. If I were Scalley I'd absolutely consider him for my DC. The Utes have a few other candidates on their defensive staff who are rumored to be in the running. Personally, I think that now that BYU is in the Big 12 it will make it easier for the Cougars to keep Hill from jumping to other DC openings.