You can say it as promoting Mess to OC, and you would be correct. But I think it is more accurately traces to Tom Herman's departure. I understand that Rice melted down over time similarly to ISU. We have yet to see if Ohio State will fully recover, but they too are clearly much worse than they were last year with basically the same players.
In retrospect, it was naïve to assume that we could promote from within to fill Herman's void. At the time, it seemed to make sense to continue the direction. But, as we learned, Herman packed up his genius and took it with him.
Viewed yet another way, when Mess fell on his face in his first year, it was a mistake not to just fire him. Instead, the mortal sin was to put good money after bad by hiring Klenakis, and installing the Pistol. In tandem with that, was the initiative to overtrain (recall the blocking sleds) the players that summer. The result was a disastrous rash of injuries from which we have only recently recovered. And with those injuries came chaos on the offensive line, and in turn, QB Sam Richardson got trampled. That destroyed his confidence over time, and he never recovered from that.
Mess year 2 (Klenakis/Pistol) is often overlooked when accounting for all the offensive changes endured by Sam. In reality, it was 5 offenses in 5 years. I can only hope that some elements of the Sturdy offense are retained in order for the younger players not to have to suffer a similar fate.