EZ-E is almost certainly correct. BYU is the Notre Dame of the Mormons. It is the most prestigious job a Mormon football coach could want.
And make no mistake. The Mormons ARE very wealthy. Incredibly so, as a church. First, no LDS coach (that I can imagine) would ever dare put their church in a bidding war over their coaching position. It would be an absolute outrage and insult to every other Mormon. Part of the job is taking a lesser (perhaps) salary than you might command elsewhere. The benefit is stability and the long, long leash.
Even if NU (or any school) went after Mendenhall, and BYU got wind of it or wanted to keep him, they would strike first with a raise and extension that would make it impossible.
I've known many LDS members, and they are just not like many other groups. They take loyalty to their church incredibly seriously, and I cannot imagine a scenario that leads to Bronco leaving unless it was mutual by both the coach and church. Or if the church made the decision, due to performance issues. But to imagine an LDS coach leaving for higher pay? Highly unlikely.