My problem with the way things went at the Sunwell is not that she was banished, as I've said before.
But the whole scene somehow makes no sense. Alleria walks close to the Sunwell and it gets disturbed enough to not only cause chaos, but to actually conjure voidbeings and a voidportal, which gets closed by using voidmagic again. And all that void-magic thrown around, void beings running through the middle of the well and Alleria's continued presence doesn't do anything to the Sunwell, no it gets better and voidmagic even helps save it. It doesn't make sense.