What Lebeau is saying about Pittsburgh is the NFL. He the sharpest knife in the drawer, but that comment misses something about college defense. Every guy on defense in the pros can, if he gets his leverage correct, stop the O-Lineman in front from moving forward. All the pro D-line must do that in order for the supermen LBs to fly into what little creases there are and stuff things. The added value of the DEs is of course when it's NOT a run. Then their ability to penetrate, loop, and pass-rush eclipses what the NT mountain can do.
Now take college ball. Many of those thoughts still apply, but college defenses have holes in them: guys who just aren't as good as pros or even their teammates. Giving the offenses more shots at you is MUCH more likely to kill you. SO, it is vital to stop the opponent on third and short and get right off the field. Lou Nix stops third and short if the opponents are stupid enough to try to run between the tackles. Unlike almost any other player, the O-Line in that area just seems to encounter an unmovable stone, and the QB or RB runs right into his lineman's butt, forward progress stops, and the guy has to try to improvise [meanwhile Te'o, Spond. Shembo, Russell, Jackson, Motta have circled in like violent vultures with other ideas].
Nix is a mountainous moving stone. HE GETS US OFF THE FIELD. We have nobody else like him [watch Springmann and Schwenke "shock" backwards a little on those plays]. No amount of tinkering-in big DEs is going to do this. The only other guy who looks like he stones the guy in front of him basically every time on our roster is Day. Sheldon is a mini-Lou. Maybe he could replace him... sort of.