Vibration seems not to be the cause, according to the HUMS data, no abnormal vibration has been detected until 2 seconds before separation.
Fatigue prediction and lifetime calculation are statistics. Taking into account scattering in materials resistance, industry take margins that keep the functionning points far from the real limit.
This kind of tragedy shouldn't happen but it's a nice dream to believe that it's all about design matters. It's a shame that the fractured gear of REDL wasn't recovered (where the crack started)...
Concerning the future of heavies, it still exists and what we know today of O&G will change.