Congestion charges are not primarily about money. The desired effect is discouraging people from driving there. The income is a nice side effect, but not the goal.
If it was about discouraging why not total ban of all non-emergency vehicles? Or maybe allow busses and trucks, but absolutely nothing else? That would solve problem much more effectively.
Or maybe first come first serve system. If too many vehicles are inside border new ones have to wait until one leaves?
Or maybe first come first serve system. If too many vehicles are inside border new ones have to wait until one leaves?