Teams do a lot of different things with their staffs sure. But IMO if McVay felt good about his passing game coordinator production he wouldn't have hired an OC. Or he would have promoted the passing game or run game coordinator to OC.
I also think the fact neither of them were promoted to OC last year is a great example of how smart McVay is in spite of his relative youth as a head coach. Lot of young head coaches would have filled the job. McVay left it open and we saw this season why he did that... Because neither of those guys were up to snuff.
Now I realize we can get into all manner of other things in this, like the OL changeover and how that would affect the aforementioned ability to do their jobs. But once again I go back to the evidence at hand which is McVay's actual decision. He knows what he wants as his coordinator and went out and got it.
