Could a number of seemingly unrelated individuals introduce a number of bugs over time to form and exploit without being detected?
Doing code review at work I am constantly catching blatantly obvious security bugs. Most developers are so happy to get the thing to work, that they don't even consider security. This is in high level languages, with a fairly small team, only internal users, and pretty simple code base. I can't imagine trying to do it for something as high stakes and complicated as the kernel. Not to mention how subtle bugs can be in C. I suspect it is impossible to distinguish incompetence from malice. So aggressively weeding out incompetence, and then forming layers of trust is the only real defense.