Kinda the sweet spot for hybrid-electric drives, no?
It is, but not with diesels, because they don't start-stop as gracefully as gassers, and probably never will — at least, not until gasoline engines eliminate their startup advantage by becoming just as high-compression as diesels. And in fact, the trend we are seeing in gasoline engines is to move towards higher-compression direct-injected designs, or to moderate-compression DI engines with turbochargers. In the bargain they are becoming just as expensive as diesel engines, because now just like the diesels they need a high-pressure fuel pump and a heavier cylinder wall to deal with the increased cylinder pressures, as well as the use of more exotic alloys to increase rigidity and durability while keeping package sizes down. So what we're seeing is that diesel and gasoline engines are effectively converging on the same point.
At some point we might even get engines which are capable of running on either cycle, or some other thing somewhere in between. Koenigsegg has a practical solenoid-actuated valve design (as well as a fully custom in-house designed PCM) that in practical terms can actually permit changing from one combustion cycle to another while the engine is running, for example switching from four-stroke to two-stroke at high RPM. Maybe one day we'll have single engines that can run on spark or compression ignition.
Or, you know, maybe we'll give up on this infernal combustion crap, and go electric. Batteries are getting better faster than ICEs.