Also if I were to give Duval some credit, I'd say the 'conditioning' part of S&C was not bad. If you take the 2021 season for example, and look at all our most physical/best opponents (OU, MICH, MSU, WISC, Cockeye, OSU)... I don't think there was a game in there where we 'wilted' in the 4th Q. IIRC we looked good physically in those games, we just lost those for other reasons (usually dumb Frost or AMart related reasons).
Duval's problem was that we had too many players who underperformed their star rating and/or did not get better in their time at NU. Not all of that is his fault but S&C bears the brunt of that.
That plus the killer injuries + the general stiffness that our players (especially linemen) seemed to move with.