Coaching TEs isn't really much in the way of position coaching. As a very wise man pointed out, TEs get coached by the Line coach for blocking technique and scheming, and by the receivers coach for catching and route running. It's why we see grad assistants do it with many programs.
We have a guy coming in, unfamiliar with the position coaches and the players, trying to use his knowledge to make an offense, he can't revamp in 3 games, successful. Even if he were to take the job as OC next year, it would be a year of 'training and getting to know the program', so being a full time QB coach might or might not be in the cards.