It astonishes me just how few people get the point of hybrids as a product. When you put a small engine in a useful sized car, contrary to uneducated assumption & manufacturer's BS, it usually results (out in the real world) in increased
not decreased fuel consumption because small engines lack torque and as a result you have to work the engine very hard, stepping on the throttle pedal (accelerator) hard more of the time. Electric motors on the other hand produce huge torque but lack power, so by combining both
you get a small, frugal engine that can be given a big boost in torque when needed, that's why Honda refer to their hybrid system as "IMA" "Integrated Motor Assist
The Prius' manual "EV Mode" (Electric Vehicle, electric power only mode) button allows you to drive at a maximum of 31 mph for a maximum
of 1.2 miles, you can't drive across town on electric only because you need approximately another half ton of batteries. BTW; That's not an exaggeration, I literally mean half a ton minimum.
Honda didn't bother with a manual electric option because only switching to "electric only" mode when the computer knows it'll do the job properly means it can switch to electric only more often because it's not continually trying to build battery charge back up from minimum because the problem between seat and steering-wheel keeps pressing the bloody EV button!
Hybrids are not electric cars and they're not meant to be electric cars! They're petrol cars that can significantly reduce fuel consumption and therefor emissions. The biggest problem with achieving that economy is, again, the problem component between seat and steering-wheel because most people don't know how to drive economically. The current Insight however uses some very clever instrumentation to help drivers understand how to drive economically and even makes it in to a game that you win by driving economically!
Not only does the Prius lack the clever psychology but if you buy the current model you're a guinea pig for the development of first gen Atkinson cycle engine technology and as anyone who owned a series 1 Mazda RX7, Toyota MR2 or a Honda with a first gen Variable Valve Timing engine can tell you, first gen engine technologies are always clever but deeply flawed.
@KiwiEV; the closest thing I can think of for the ultimate ideal you described for now would be any Mitsubishi with a Japanese market MIVEC V6 engine, which allows you to turn 2 cylinders off for town driving, not a hybrid.