Some things to remember:
1) With 1:29 left in the 3rd quarter, this drive was the first time BYU had crossed the 50 in the half. There were no guarantees that BYU would get very many more opportunities to score.
2) According to the "analytics" with the down, distance, and field position (4th and 2, inside the 10), the chance of a successful conversion is 55%. In other words, only slightly better than a coin flip. (See graph below)
3) Although momentum looked to be swinging towards BYU that drive, USC had just stuffed Williams for a 1 yard loss at 3rd and 1 the previous play. Another stop would energize USC (this is what happened - they marched down and scored a touchdown the very next possession).
Given all of that, I think he should have kicked the FG.