electric/batteries works great for start/stops. For highway cruising, which 100 miles range would probably include a lot, internal combustion engine isn't that bad. I'd expect that there isn't much practical difference efficiency-wise between 20-30 miles battery range hybrid and a 100-miles range one, especially considering that the latter would be carrying additional at least 200kg of batteries.
I think we will start to see cars' internal combustion engines which are specifically targeted to work only as generators, i.e. highest efficiency only for specific rpm value - current ICEs available are "wide rpm and load range" designed to be a primary engine of a car. Volt (and Karma) as far as i understand have such generic engine, not a "generator" one. Dropping a lot of old requirements and having the ability to match generator to the engine, we may see very different engine types coming in which would match new requirements much better.