He is going to one of the very top level NCAA coaching positions in the nation and, I assume, one of the very best paying.
It is a a little silly, IMO, to to think a coach should stay at BYU in this scenario. It's not like BYU won't fire a coach just because the coach really loves BYU, after all. It's a job. And one that pays waaaaay too much for things like loyalty to rule the day.