Sans,
Any idea if the transmission problem is directly related to the ABC concept? Simplistic analysis would say as you grow an ABC in size the rotor moments (which are already in the extreme on smaller versions) go up significantly while Xmsn case stiffness goes down. At some point the mast bearings can’t handle the loads or deflections become so high the case breaks or gear alignment is impacted.
Or is it they just can’t manufacture gearboxes like on the CH-53K?