There are too many unknowns for any of us to know what the correct move is regarding Rees. I was trying to highlight the evidence that suggests he can adjust to what the defense is doing in real time and get the team into winning plays. This is where his experience as a player helps us understand his ability.
As for the way he sees the game and it filtering into his coaching, one of the criticisms of Book has been his unwillingness to take chances, very much the opposite of Rees. Rees has been his position coach for three seasons now.