Back in the early and mid 1970"s a bunch of work started on trying to make roller transmissions, but the extreme pressures and the need for close tolerances with new welding techniques proved their downfall, I think.
The first program I worked on at Sikorsky was a roller gearbox one.
Modern transmissions with double helix and split torque are very light and very reliable. For helicopters, which favor relatively constant speed, the advantage of roller or belt drives is somewhat lost.
The automotive variable ratio transmissions allow one to do away with clutches and gears, because they allow almost continuous change in gear ratio while still driving, so that the car's wheels can be driven by the engine throughout the speed range, in spite of the fact that the engine wants to be in a narrow rpm band. The rollers or belts are shifted down variable diameter shafts so that smooth continuous drive is had, a real advantage.
Wear and maintenance issues are still an issue, but materials are coming that might solve this. Of course, electric drive also is transmission-less, and might take over.