I think the problem is that below the 80% level (or equivalent on other helos) there is insufficient autorotative force being generated by the blades - they won't stall immediately but will continue to slow down and, much below the 80% level, even lowering the lever and manoeuvering won't recover the Nr again if the engine has quit for real.
A teetering head helo can't cone up and clap hands, the aircraft just falls from the sky with ever slowing rotors meaking the AAIB investigators job messy but straightforward since the fuselage is squashed flat but the blades are intact.