The endowment has nothing to do with athletics. The AD has its own budget. I am also guessing there are bylaws in place that restrict expenses being paid out of an endowment.
There are a few reasons why a coach/employee might agree to take a temporary paycut. First, it shows you are a team player, unified in the effort to make it work, and would consider not letting lower level employees get laid off. Two, the coaches are on contracts, and when this is made public, it very well could effect future job status. If your performance is mediocre, then other factors are considered when offering deals. Kind of a good will type thing. I wouldn't call the personal publicity as "positive," so there is more to it than just not receiving 10% of your pay.
Your argument is better suited for coaches like Osovet, Artis, etc. Jim Chaney has made plenty of money, and there are others who probably could have sacrificed a bit.