The other thing is that position coaches and even coordinators can have blind spots. You really like a kid and see he has such potential, but when he gets on the field he makes the same mistakes over and over again. KF has to look at the bigger picture and help his assistants see things they may not want to see.
That all said, I cannot imagine a universe in which the OC says, I 100% believe in this guy as our starting QB, and KF says, "nah, play the other guy." If that happens, you have the wrong OC. You gotta let the chef pick his own ingredients, and I believe that KF is the kind of coach that might try to persuade his coordinators who to play, but not order them to do so against their will.