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I'd be incredibly surprised if the majority of modern cars were most efficient below 3000 RPM. Maybe pushrod V8s, but DOHC 4s are a different animal entirely. And spark-ignition engines are typically most efficient near (but not at) wide-open throttle.
The (thermodynamic) efficiency of an engine at idle is not particularly important. The fuel efficiency of the vehicle containing that engine is zero regardless; an idling engine isn't moving the vehicle.
CVTs have long had a small niche in the economy segment of the market. The two big problems they have is that the rubber belts they use can't handle a lot of power, and they cause poor throttle response and even worse perceived throttle response (you push the pedal, hear the engine rev up, and THEN the car speeds up). Audi has a steel-belted electronically controlled CVT which supposedly solves both these problems, but it's only available on a few models. It's unlikely CVTs will be obseleted by hybrids, as often CVTs are paired with hybrids.
As far as I know, no one has yet managed to get a practical _serial_ hybrid (like that in a locomotive) into a car.
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