We've seen this kind of structural problem before in the pylon pins in B747 freighters
When you start designing load redundancies in multiple load paths you may miss a handoff of what path is now carrying the load and by how much vs flight operations. The assumed response is to cover this by high margins of safety over normal conditions.
Sometimes (rarely) some hidden condition pops up and creates abnormal stress conditions which now over many flights could result in early fatigue. Again even with fatigue cracking the concept still is failsafe as long as nothing else starts to fail.. The airworthiness action in this case addresses increased inspection early enough (inspections etal.) to maintain sufficient margins of flight safety.. Yes it is a pain-in-the a$$ for the operators but it doesn't always require an FAA design review or complete replacement of the design.