First, I guarantee that that wasn't the only mistake (if we concede a mistake) made by the refs that day. Second,
that wasn't the last play of the game. Assuming the game doesn't change from that point on with a different call is a mistake. How knows how it would have played out if that score would have counted. Bottom line is stuff like this happens all the time, and it's unfair hindsight to conclude that BYU would not have won the game but for a blown call.