I think Bronco just looks at the overall burden of the job, and is taking what former Florida & current South Carolina Head Coach Steve Spurrier said a few years ago about staying with a program too long and why he chose to leave Florida when he did; "10-15 years at one prograom is long enough." I don't think you'll ever see a LaVell Edwards, Bobby Bowden or Joe Paterno length of stay, even at a program like BYU.
Look at the three above I've mentioned and their lack of exit strategy:
---Don't like changing assistant coaches, at least very often later in carear (Edwards).
---Both Bowden and Paterno still think they have another national championship, despit being eightyish and not telling recruits they'll be there after 4-5 years when theri eligibility runs out.
I think Bronco looks at this and wants to keep a credible exit strategy in mind. Big money won't draw him away from BYU since BYU now pays head coaches what his average/median counterparts in the PAC-10 or Big-12 (with the exception of Oklahoma's Bob Stoops--$$3.5 million) typically make. As it stands Bronco makes as much as Gary Patterson does at TCU, and those two are the highest paid MWC coaches, the same as Pat Hill over at Fresno State of the WAC. What SMU is willing to pay June Jones ($1.8-$2 million per year!) could push renegotiations for the three mentioned higher. What I read is Bronco could have an exit stretegy in play, and sometime between age 50-55 he could just fade away as an associate AD and setting up a horse ranch on Maui, and have a worthy successor in place that pleases him, the fans and the administration.