Having flown many thousands of hours in offshore helicopters some decades ago I was truly shocked to see the video evidence of this accident. The whole rotor head with intact blades separated from the gearbox and came down like a sycamore leaf. A structural failure of this magnitude is non survivable in a helicopter.
Also having been involved with the Puma accident in Sarawak in 1980 when the gearbox failed on 9M-SSC I don't recall the main rotor detaching like this. One or two blades detached through the shear deceleration forces and the captains door flying up into a blade when he released the door but otherwise the rotor head was still connected.
This looks like a failure of the rotor head itself (jesus nut?) for the whole rotor system to detach so cleanly.