If your argument is that ToD part 3 is too hard because of banish then yes lowering the risk would be the goal. However if your argument is that banish is a bad mechanic then you need something to off set the change in risk in order to keep the difficulty of the raid the same.
As for arguments against the change if you leave the DC same you would likely reduce the time needed to kill horoth to 1/2 or 1/3 making part 3 trivial.