I do not agree in general, but I admit to not knowing the details of that system. Those of us in the industry have been waiting for 737 rudders to fail for a long time. The data was there. This is the same as waiting for a pilot to rip the tail off an airplane as happened over New York. The certification basis of those aircraft was not in harmony with the information the operators used to fly them and gravely distorted the engineering that had gone into designing the aircraft.

In both cases, there was a certain inevitability given the processes at work, the conditions, the changes and the difference between engineering reality and what is presented in the FAA Type Certification process as the basis for the modification of the Type Certificate. As an engineer working on aircraft, one thing is for certain, engineering data is laundered to the bare minimum in the interests of certification. This is in contrast to the engineering NASA does. The cert process does not achieve transparency, as it is a legal not an engineering process.

One can blame manufacturers, but there really is no way for them to be transparent and ever ship anything (ref. Lear Fan). I am not, however, advocating retracting any responsibility from Boeing. They must bear the responsibility for their product. I am only saying that we have created an environment as a country that makes it unlikely we will see the behavior we desire from manufacturers. As a person who likes to land the same number of times I take off, my interest is in helping the government improve as well as the manufacturers.

So Boeing decided to conclude that the existing system was sufficient. And they proved it to the FAA. There is no precedent or standard applied to the parts of the airplane that are not changed, once they are deemed not to need changing. This is similar to the housing code. Replace a socket in a house with knob-and-spool wiring, and you do not need to fix a thing. Rewire a bedroom in a house with knob and spool wiring, and the code will not let you put new knob and spool wiring in, one must then bring the house's electrical system up to code.

The reason there still is a 737 is the tremendous economic benefit afforded to manufacturers who use safety analysis to show that the previous iteration of the aircraft's systems are sufficient given proposed changes on the aircraft. That's why it has ridiculous engine nacelles, ridiculous landing gear, in many 737's ridiculous avionics. A new airframe is required, but the cost of a new airframe's certification is much greater than the hacky re-certification of an old type certificate obtained when standards and processes were very different and much out of date.