We will see just how good USF is, but their offense dominated BYU in the second half. Losing starters completely unwinding a Tuiaki defense is a oft repeated disaster. Your point about a lack of depth/development/ performance by second string players is critical. Either Tuiaki and staff can't recruit players to fill a roster or they are not putting enough effort into having them ready to step in.
All of that is on the coaching staff. Tuiaki won games 2 and 3 with the defense this year. Really great stuff....but last night they were a pale shadow of weeks before.