What if turrets had a threat system instead of "who attacked first"

Way I would do it is that turrets would work as they do right now, but if their current target hasn't damaged or cced a friendly champion for 2 (maybe 3) seconds, the turret will switch to the most recent enemy champion that has damaged/cced a friendly champion. That way, a tank can't just sit at the edge of aggro range and let their teammate have a free pass under the enemy tower.