It’s not just his position’s performance though.
John Butler spent the last 10 years a defensive backs coach in the NFL. Prior to that, he spent ONE season as a defensive coordinator in college.
Nebraska has the budget to hire a tenured, excellent college DC. Why would you give a guy the reigns as DC who only has one year of experience in that position and that one year was over 10 years ago? Secondly, the guy has been in the NFL for the last 10 years, perhaps his position group struggled some because the coach is learning that teaching professionals is different than college kids?
A promotion for Butler doesn’t make sense at all. Go try and get a proven guy first