I think Kuehnja was held "prisoner of the moment" by watching Grace/Fox/Calabrese drunkenly-stumble their way after Purdues scat back off the edge all night.
There is plenty of athleticism on the defense. I've seen a couple critical of Russell, but there was one play where Purdue's back broke off a big run around the end and Russell had enough athleticism to flatten off rather than give up extra yards via taking a deep angle, and still had the speed/acceleration to push him out of bounds.
BJax is fine, Farley isn't strong enough (speed is not the issue) and Collinsworth/Shumate are not burners, but they aren't getting BURNT either (in fact, the one time they've been beat on a "go-route" was due to an UNDERTHROWN ball, ala Gary Gray). Even Jaylon, Councell, and Prince look like supreme athletes with good speed for LBs... The issue is Grace/Fox/Carlo. They are supposed to go sideline-to-sideline, but we were far more spoiled by Manti than we knew. They look like bumbling idiots when chasing scat backs, but that is also a matter of scheme.
Once Diaco called for more nickel, Cole Luke was very impressive (at least on a few plays) in hanging with the backs and slot receivers.