As usual, I think you're over-stating the fragility of disc brakes. In my experience, it's difficult to bend a 160mm brake disc to the point where it becomes unusable without also causing serious damage to the surrounding components (ex: wheel, fork, frame, rider). The best I've been able to do in my numerous mountain bike crashes, was bend the brake rotor to the point where it rubbed the pads a bit... just like an out-of-true wheel tends to rub against rim brake pads. In both cases, braking performance suffers. Luckily, most bikes have two brakes.