Rear Suspension Springs problems of the 2003 Toyota Corolla

I own an '03 Toyota Corolla which I purchased new. At the 45,000 mile mark my rear suspension collapsed while driving at 20mph on a newly paved city street - there were no potholes or obstructions. The service department at my Toyota dealership attributed the failure to be "climate but no excuses. " the rear suspension had "rotted" (rusted) out. They also said that this was not all common. I just spoke to a Toyota mechanic with 20 years experience - he has never seen this problem but said it was possible. He also questioned whether the underbody had been properly treated on the assembly line. He would not expect this to happen in such a young car - more likely with a 10 year old car. I did not get to visually see the broken, rotted parts. Toyota repaired the problem at my expense - $604 - and they recently offered me a settlement for $301. The parts replaced were the rear struts and springs. This issue also raises concern for the front suspension.