[jerf]

The problem with engineering defense in depth is that systems naturally evolve to "spend" that defense in depth. Constant, active vigilance is required to maintain a culture that honors and preserves defense in depth rather than exploiting it. It isn't that hard to believe that Boeing has failed to preserve that culture. A company like Boeing would be constantly fighting against multiple forces pushing them to exploit the defense in depth rather than maintain it. Some of them, like the Harvard MBA mindset, are very difficult for large companies to resist. Short term costs that result in long term benefits are a hard sell to almost anyone, but current American business culture is definitely not strong against that.