the guy did very well with Herbert. Wouldn't make sense to come in as a new HC and disrupt what seems like an already good marriage. I'd come in and make the transition as easy as possible, leaving a comfort zone for my rookie QB, he seemed to do very well with the system in place.
I know a new HC likes to do things his way, but being a true rookie HC it seems logical to not come in and change over everything...especially if the QB you have in hand is coming off a great season and add to that he is going into just his 2nd year.....I would keep his curve going the way it is and not bring in another system year 2.