I would disagree. A lot of the Rhoads-era defenses were passable or even good.
It was the offense that was sniffing glue.
I remember one game where they didn't earn a first down until well into the second quarter and the whole crowd at Jack Trice erupted in the most backhanded and sarcastic manner.
Thing was, those bad offenses would eventually drag down the defense. They would wear down after too many snaps on the field with too little rest between series and offenses would pulverize them with runs to desiccate the clock because they had the lead and were looking to end the game.
So yeah, Wally is pretty far down my list of people to blame for the second half of the Rhoads era.