Or are some metals impervious?
> The fatigue limit or endurance limit is the stress level below which an infinite number of loading cycles can be applied to a material without causing fatigue failure.[1] Some metals such as ferrous alloys and titanium alloys have a distinct limit,[2] whereas others such as aluminium and copper do not and will eventually fail even from small stress amplitudes.
> At the time of the accident, N259UP had accumulated a total time of about 92,992 hours and 21,043 cycles [..] A special detailed inspection (SDI) of the left pylon aft mount lugs would have been due at 29,200 cycles and of the left wing clevis support would have been due at 28,000 cycles
This isn't talking about replacement, only inspection; and it wasn't going to happen in the near future: 7k cycles at four flights/day means inspection is due in 5 years.
It wasn't doing four flights per day. As a long-distance cargo aircraft it was doing two flights per day, and I doubt it was flying every single day of the week.
So we are talking about at least 10 years before that inspection was due.