Structural components aren't likely to be serviced by front-line units. If they were suspected of an issue, it may gain an increased inspection interval (C-130 wings come to mind).
If this longeron was installed during manufacture, there's no way to catch it. If it was installed as part of a life-cycle update at some point, it would have most likely been at depot-level maintenance and again, the front-line units would have no reason to inspect it.
If it was an inferior part manufactured by a sub-contractor, than the primary contractor (McDonnell-Douglas -> Boeing) is responsible for ensuring all sub-contracted parts meet spec.
If this part is indeed out of spec, and M-D/Boeing had oversight, I believe there is a liability that may be exploited here.