The current Rams OC just got landed a head job in the league. Guys touching/under/associated with Shanahan and McVay have done quite well with their careers. Rees going to work for McVay, even if he isn't technically calling the plays, would be good for his career.
Rees was going to have to leave the Burnt Ends shadow for career development anyway. The pro route was probably more likely anyway, as he could still get "promoted" without getting a head gig yet (he's only 29). Working with somebody like Shanahan/McVay/LaFleur seemed like perhaps the best way to broaden his expanse, hone his chops, and move on from ND OC without having to either make a lateral move or take a HC job at a crappy program. If Rees were to work for McVay for 2-3 years and do well, he would probably be able to pick his next job- at the ncaa or nfl level.