I think everyone has to remember that when a disk fails it will almost certainly wipe out something important. Fuel, wiring, hydraulics, control cables, structural members, other engines, the drinks trolley.. all critical systems that it could destroy.
Does that mean we should make everything twice as redundant and armour the lot? No, because the idea is that the disk should never fail in the first place!
Completely agree... the thing is, I think if I was among 500 other passengers, I would think that someone had already thought of it. There a load of What If's...and unfortunately each of those What If's have a cost associated with them. I bet that if you were the bean counter on a plane in trouble, you would be having second guesses about what you didn't agree to as necessary to spend on a back up system if it would have helped you in the situation you are in.
I think as well, in this case, they had a part of the engine that they thought they would not need to contain, but it failed. What i guess worries me more about this situation is the fact that it could have potentially caused alot more issues than it did.