"....The accident happened when the main rotor broke off, as a result gearbox failure, and severed the aircraft's tail boom....."
The description of the accident in the article seems a bit silly. The "main rotor broke off.....and severed the aircraft's tail boom". Once the main rotor separated from the aircraft, what significance does the impact with the tail boom have? Are we to believe that the aircraft would have been able to maintain stable flight had the departing main rotor not severed the tail boom?
Catastrophic structural failures of main rotor shafts/bearings should be an extremely rare event if modern design, manufacturing QA, and maintenance procedures are followed. Long before that main rotor shaft suffered structural failure, the MRGB HUMS should have given an indication of a problem.