I like KF & BF very much so my comments are purely something I have believed for a long time and that is our offensive lines have been very solid but not elite. We have been super at the tackles as you noted and I agree. It is my belief that our line play is not elite because we often, not always, play too many small stature guys. Yes, they are really coached up and the staff does a great job but the proof is in the pudding and the facts back up this argument.
First what was the best O-Line under the KF era and subsequently the best offense? Answer we all know is 2002. The smallest guy on that line was Bruce Nelson at 6'4" and 300 lbs. Coincidence?
Second how many times have we finished in the top 3 in the conference in rushing the football (B1G stats only)? I throw out the non-con because of the patsies. Compare that to Wisky over the same period.
Third how many interior lineman (guard, center) have we had drafted under KF? Compare that to the tackles.
Fourth take a look at the consistent top 20 programs in major college football and look at their starting O-lines. See a pattern?
Fifth there is a reason the NFL does not draft small lineman. They know what works over the long haul.
I do not buy for a second that in today's age we cannot find 5 guys who are 6'5" and 300 lbs who can also move. You might be able to find this in the state of Iowa alone.
To me there is nothing more important to offensive football than having a great and dominating O-line. I just think KF is stubborn and thinks he can turn undersized guys into productive interior o-lineman. Sometimes it works but not always and possibly not when we need a 1-2 yards the most.