First, I don't portend to believe we'd get a 100% straight answer from that question.
Second, the uncomfortable truth is that it was a bad coaching decision, and there was as much of that going around, especially at the end of the year, as there were bad attitudes from the players. The problem is, some people want to believe Saban is/was perfect, just like Bryant was perfect, but in neither case is it true. In regards to the OU game specifically, it's not like the Bama offense hadn't shown the flexibility to use tight ends as blockers on the left side before. When Travis McCall and Nick Walker were playing together early in Saban's tenure, it happened on a regular basis.
Perhaps this was Nussmeier's call and that was part of the reason he was allowed to leave, I don't know. I'm hoping so, that it was Nussmeier who failed to properly assess the threat, rather than the alternative, which is arrogance or stubbornness -- "by gawd, Kouandjio ought to be able to block anybody, we ain't changin' nothin'."
What makes me most optimistic is that I don't think all this talk of a system reboot is hot air. I think Saban figured out where his problems were and made the appropriate changes. We'll see how much harder the team appears to play, because as Saban noted this week, there are positions at which the talent isn't what it's been in recent years (which I took to mean OL, QB and DB) and people are going to have to work harder to make up for it.