The issue here isn’t about airframe cracking in general, which is fairly normal and expected in the lifetime of the plane, but cracking in the “pickle fork” in

The issue here isn’t about airframe cracking in general, which is fairly normal and expected in the lifetime of the plane, but cracking in the “pickle fork” in particular— which is abnormal, as these are not normally expected to crack during the plane’s lifetime according to Boeing’s own modelling and expectations — engineers had rated these at 90,000 cycles, the effective lifetime of the craft. This appears to indicate (yet another) QC and manufacturing deficiency (Boeing already is dealing with other such issues with the 787 and KC-46). The claim by OP that this is normal because aluminum cracking is normal is really wide of the facts. This part was engineered to withstand the forces it experiences for a certain timespan, and the fact that some now appear to be failing in less than one-third of the lifespan the materials engineers expected it to calls into question whether Boeing or a subcontractor is failing to engineer these components to spec.