The left rear lower control arm inner bushing is the one that looks like it is coming apart, the right side looks good in comparison. The bushing can be replaced without necessarily replacing the whole arm; if someone is creative they could potentially do it without removing the control arm from the car. Why do they want to replace the knuckle; bushings worn out there? They can also be replaced. The bearing assembly is not pressed ito the knuckle; it may seem like it when trying to remove it because of rust. At 100,000 miles, I wouldn't expect the bearings to be bad.