The Delta fans Bitmain uses are DESIGNED for long life at 100% fan speed.
Running those Delta fans 24/7/365 at 100% is no problem. We have 74 S9's and we find that sometimes the fan blades will become brittle over a long periods of time, 12-18 months. Very occasionally (it's has happened 3 or 4 times) one will 'explode' in rather dramatic fashion, meaning that a blade will chip or break and shread the rest of the fan rattling around inside the cage. The fan will stop spinning and the miner, no longer being cooled properly, will perform a thermal shutdown within a few minutes, and the downed miner will pop right up on our monitoring software. Replacement fans are cheap and take less than 5 minutes to swap out, and we've never had a miner suffer any damage when this occurs.