Why don't helicopter manufacturers build in a one way sprag clutch into the main rotor shaft, so that in the event of a gearbox seizure, the output shaft keeps turning in autorotation?
Well, two thoughts:
Seizure of gears is one thing, but it's the seizure of the main transmission bearings which is also a very big factor, and the use of a sprag clutch won't help that situation at all. Were sprag clutch to be contemplated, it would have to be immense to transmit all of that torque at the relatively low main rotor RPM, and thus be very heavy, and probably more failure prone than the weaknesses it was there to overcome!