Maybe he "left" because they were being generous about not openly shit canning him. 9 years in one of shittiest defensive schemes? Bye. It didn't look like Pettine had much say about coaches under McVince.
The other thing is a 1st time HC will always have issues bringing in experienced position coaches. It's kind of funny thinking about how Capers came on board when McVince started. Nobody wanted him after working as a consultant with the Patriots. The parallel is a bit interesting as well but at minimum Pettine isn't on the cusp of obsolence in the first place.
I'm sure the overall struggles with building a cohesive staff are part and parcel for newbie coaches and staff. I can only imagine they're finding out who was actually worth two shits.