I get how regular brake pads wear out as they are repeatedly clamped against rotors turning at high RPM to slow down and/or stop 2000lbs or more of vehicle moving at speed. Friction like that takes its toll.
But parking brakes get set while vehicle is stationary and once set just essentially stay there. Yes, some friction is involved, but it is of a static/non-moving nature, so I don't see how they wear out. What am I not understanding?
One thing that can appear as parking brake wear is actually the stretching of the parking brake cable.
Every time you apply the parking brake, the cable will stretch just a tiny bit. Every so often, the cable should be adjusted, but few people actually do that. Eventually, the cable is stretched to the point where you need to pull the parking brake handle all the way to its mechanical stop but it's barely applying the brake, thus giving the impression that the brake is worn (especially if you're parked on a hill and the brakes aren't applied hard enough to stop the car from moving).
For 1 and 4 above, it is obvious that the single set of pads or shoes will wear as the foot brake is used.
For 2 and 3 above, under normal operation, the foot brake pads should wear out and the parking brake pads/shoes shouldn't get any wear, since they are only in contact with the disk when stationary.
Wear can occur on the parking brake friction material if the vehicle is driven by mistake with the parking brake engaged partially or fully, but unless this is done often, then the wear shouldn't be significant.
In point 3 above, where there is a different surface that parking brake shoes/pads press against, the metal surface wont be getting cleaned/derusted under normal operation of the brakes. Rust can build up and abrade the pads/shoes while driving, causing wear.
Additionally the parking brake friction material, while not being used to slow the vehicle down causing wear, are still in a hostile environment with constant changes in heat. This can cause the friction material to break down and eventually require replacing.
Build up gunk from brake dust or other deposits also affects the operation of every type of brake, also an eventual separate parking brake. While technically not a wear factor, it requires maintenance from time to time.
The mechanic tasked with repairing or maintaining the car is well aware of that. Most of the time it is cheaper to just replace the friction material than cleaning it, especially since he needs to open the entire brake anyway.
While technically the brakes might be still sound, he also needs to explain why he left an eventually decade old brake in the car while still billing the customer.
Therefore he just replaces all that might be in bad shape and bills the customer for a worn brake.
With age, the parking brake flexible cables, and the mechanical linkages that operate the shoes, get dirty and corroded, and any return springs get weaker. Then the mechanism won't back off completely, and the brake drags slightly. That generates excess heat, which makes the situation worse fairly quickly.
