That is a really great question. I think that if keeping coaching salaries down could have been agreed upon as a good thing by the NCAA, I would have suggested that the rules would have needed to be tied to the pay scale of the university. Perhaps, that a school could not pay a head coach more than something like 80% of the chancellor, president of the school, or that the coach could not be paid more than the average salary of the top ten percent of tenured faculty, or that a an AD had to make more than coaches and that an AD is paid at a Dean level and head coaches at a tenured prof level. I am not saying this would have worked. But, getting coaching salaries more in line with what the university or college pays others would make more sense than a salary cap. Some limits on outside income would be necessary if you really wanted to keep coaching compensation under control.
The problem is that for years universities allowed costs to rise due to exorbitant coaching contracts, facilities competition that led to the ridiculous, and bloated athletic department bureaucracies. Now that athletes have won the right to get their share of this overspending we are in a crisis. The budget to compete has outgrown ticket sales, bowl and game contract revenue, alumni/booster contributions and advertisement. It is completely dependent on money from those who buy the media rights to the product. This could have been avoided with better cost controls from fifty years ago until now. It will be hard to fix this now. It would require schools leaving the "big boys" and starting over with these spending controls in place. Boards of trustees and college presidents would have to make these decisions; AD's and coaches will never voluntarily admit their revenue share should shrink.