What strikes me is that it looks like there has been no apparent warning of a main rotor or MGB failure. Since the L2 gear boxes failures, early detection systems improvment (like position of chip detectors) seem to cover all aspects of a catastrophic MGB or main rotor failure (... sorry for beiing inacurate, but althought not qualified on type my company operates a number of them)
If there had been such a warning, I suppose the procedure would have been to reduce the power and/or descend to a "minimum safe altitude" ... In this case, the initial descend seems to be rather ATC related.