That sounds like what the Chevy Volt did back in the day. Turns out that it just was not feasible to achieve higher efficiency through the generator when cruising on the highway than just direct driving the wheels.
Almost certainly why nearly all hybrids have been parallel hybrids up to now. What is changing, I think, is that a significant number of people are warming to the idea of a BEV, and want all of the benefits of that, but want to fall back on gasoline in a pinch. Thus EREV, or series hybrid, which provides that crutch. Expensive, though.
