If the defense continues to play well, but we lose games because of poor offensive performance, Diaco will stay even if Riley goes. Most new coaches, unless they are married to a defensive scheme, would want to maintain some stability with the staff, and if the defense is trending in the right direction, then why mess up that trajectory.
Now there is a lot of football left to be played. If Nebraska splits the next 2 games, I could very well seeing them end the season at 7-5 or 8-4. Now at 8-4 they would have to have only 2 losses in conference play, and if they beat Wisconsin but lose to OSU and Penn State, that would give them a good chance of still winning the BIG West. If they finish 7-5 and do not win the West, I think the new AD could make the case for Riley to go. If they finish 6-6 or worse, I think it's a no brainer Riley is gone.