A designer/engineer cannot reasonably account for every possibility. There is always an Achilles heal in any design, many times multiple weak links. Additionally it is not always as simple as shutting down some systems and doing the high dive. Also, plenty of catastrophic events that have brought down plenty of aircraft with mechanical flight controls. In the big picture, airframe manufacturers, insurance underwriters, Etc. all know there will be X amount of incidents and hull losses before an aircraft is even built. However unfortunate, for the most part, it is all expected and designed into the "system". At some point everything comes down to money...