The issue I have with the onside kick, is that even a surprise onside kick should be based on your circumstances. The circumstances didn't match the move in my opinion. To perform an onside kick as a surprise, our defense needed to be playing a much tighter game and the offense would have needed that extra possession to seal the deal. If we just boot the ball downfield , we give Northwestern the whole field to string together a drive, which they proved they really couldn't do consistently. NW did have success moving the ball 25-40 yards each possession, so we gave them a touchdown on that drive.
My other reasoning is that if we lose to NW and we never tried the onside up 28-17, you would not have heard one fan say, "If we would have done an onside kick after that last score we would have won! Frost is dumb for not trying that". Frost made a decision that put the little momentum and positives we had going for us in jeopardy, and it bit us in the a$$.