97 E420 Sport Bushing/Control Arm Problems

I recently took my 97 E420 Sport to a local allignment shop to have a 4 wheel allignment job done. They technician said that he wasn't able to perform the alignment because one of the control arm bushings on the rear driver side wheel was broken. He showed me how much the wheel wobbled while on the lift. I took my car to the dealer to have it checked out. They told me they couldn't just replace the bushing and the whole arm would need to be replaced! Is this true? Also, the mechanic I talk to told me that the bushings went because of my low profile tires and big rims. But these are the stock AMG 17" rims and stock sized tired. 235/45 17. How can he blame the failure of the bushing on my wheels/tire combo if the car came like that? I was told the arms on both sides need to be replaced. Going to come out to over 700 dollars. But the mechanic told me I could just leave it. Some advice would be much appreciated.