Very good point. In huge systems such as a car everybody can not know everything. But in this case, if the ignition switch team does not know how their component affects everything about the car or how important it is the responsibility should move up to whoever writes the specs on the ignition switch. First of all for designing a system with a single point of failure in the first place, secondly for not loudly notifying ignition switch team that their component is vitally important and a single point of failure. As i also said in a post above, way too much attention is given to the faulty switch when this is actually a much larger system design error.