I agree that it's virtually impossible for a cable to saw or abrade through the shell. As I posted earlier, it took my FD wire over 10,000 miles to work through the paint.

But it is conceivable for a wire to chemically etch it's way through. If you've ever stored steel outdoors, you know the effect. You get rust where there's imperfect contact which traps water causing rust line. So once the wire is through the paint it'll hold water (salt water if ridden in the winter) against the shell, causing rust. With some use the cable will rub off the rust layer exposing fresh steel and setting up the next rust cycle. Do enough of these rust/rub cycles and it'll etch it's way through the shell much faster than would be possible through mechanical abrasion.