Assistant coaches are going to leave. It's the nature of the business and however delicate people's football sensibilities are, it's just the fact of the matter. McCullough is likely going to get some interest for G5 HC jobs, and if he has HC ambitions, he's gotta go. Nobody is getting any younger.
It is with that being said that Notre Dame OC is going to be an attractive position for a lot of external candidates. They can open this job up again and having learned from their mistakes last year give it another go with finding someone to replace Rees, which is essentially what they'd be doing in my mind.