That is not in fact how to design safe airplanes. Airplanes are safe mainly because they don't crash. Making everything redundant in an effective way is quite nontrivial and probably depends on the specific design of the aircraft (if you disagree then provide a (rough) algorithm for how to make everything redundant in a way that actually achieves the safety levels of modern aircraft). More importantly, redundancy is not particularly helpful if we don't already know that the plane is guaranteed to fly properly in a wide range of operating conditions that are precisely enough quantified that we can determine in advance whether to ground a plane because e.g. the weather is too extreme.