The dealer is totally correct, my brother did high end autobody work for many years. I backed my Eclipse into a fence post and had damage on the rear quarter panel also. That panel is welded to the frame, to cut it and weld on a new one they will use more bondo for sure! Today they can pop most of the panel dent out except the peek of the panel stretch. Then they fill in that small spot at the deepest point with bondo and go.
No matter what you do it will never be like factory when it comes to the rear quarters of the car. Had you hit the front fender or hood then a replacement is more viable! Good Luck
