Seems it'd be like a master cylinder seal failing when you push the pedal to the floor. Years of replacing pads when the dealer says to; i.e., lots of life left, then letting the pad wear to the rivets exposes the seal to a section of piston that's corroded from fluid that hasn't been changed in a while. You're right, this scenario means caliper seals would leak all the time.

Sixto
87 300D