It looks to me on the surface that there was a breakdown in the safety analysis by Boeing.
Typically design engineers address all kinds of failure conditions and are just as good at assuming that all kinds of mitigating features that reinforce the idea that it's still safe.
it's up to the flight test program to validate these assumptions. Somewhere along this line there was break.
I can't believe that the FAA oversight has the knowledge to fulfill this gap, other than themselves developing a flight test program that is unworkable with the resources at hand. The best the regulator can do is to detect any breaks in the Manufacturers safety systems
I can't see how the regulator can manage this except by reviewing the safety systems of the manufacturer in a test validation program