Heavy rust on the brake rotor’s working surface is a frequent cause for replacement around here. It caused heavy transfer from the pad material to the rotor surface and makes it look like the rotors are deeply grooved, they are not, it is the pad material that has transferred to them that is grooved. I found this out by accident quite a few years ago. I did a brake juo on a Valiant and set the old rotors on a shelf in my garage and forgot about them. when I noticed them some years later I picked one up and dropped it. When I looked at it a large chunk of the pad material had come off leaving a flat but rusty surface underneath.