I wouldn't be too quick to lash into him. Birmingham will not be challenging for promotion in any shape or form next season, and might be drawn into a relegation battle. So that would be too seasons before he's looking at a Premier League tilt anyway.
Rangers under Warburton could blow away the Championship next season, nay, they should do it. So for the start of the 2016 season, Kiernan more than likely will be playing for one of the best supported clubs in Europe, and in the top flight on a regular basis. Chances are if they get off to a flyer and he's playing well, he'll be looked at by top half Scottish PL teams.
I have to be honest, I think it's worth the risk.