If the coaches voluntarily agreed to the price, then they are not underpaid.

What you're talking about is market averages. We are paying well below the mean for coaches salaries. Often not a good idea in a competitive market. Employees paid below the mean, unless they have a particularly strong reason to stay with that employer, have incentive to seek employment elsewhere.

Our current coaches are not underpaid. But, to your point, perhaps we should be willing to pay for higher-value coaches. But BYU has historically paid well below the mean for most of its positions. A lot of people are willing to take lower-than-average wages to work at BYU.