Discs aren't impervious to damage. If anything they are slightly more delicate than rims. A disc requires hyper-straight rotors to continue working. Even a slight bend to the rotor will cause rubbing at the very least. Stuff happens out in the real world that could easily result in hub mounted discs being completely unworkable.

You can't just open up the space between the pads and ride the bike as you can with a rim brake. First, there's very little room between the pads and if you open the pad even a little the braking suffers. The pads have to be kept very, very, very close to the rotor. A millimeter gap on either side of the rotor is a huge gap. The calipers of a rim brake will tend to track the brake surface on a rim so that if you have a huge wobble in the rim, like a broken spoke, the pads will move with the wobble and the brakes on the bike will continue to work. With a hub mounted disc, the caliper is rigidly mounted to the frame and won't track any wobble in the disc. The disc will strike the pad on a wobble and do what the pads do, slow down the disc.

A wobbly disc rotor can be repaired and you might even be able to repair it on the road but it's a fairly tedious process that's even more tedious than truing a wheel...if that's possible.