The revolution has already begun. This will be the first certified electric LSA.
YouTube - Yuneec International E430 Electric Aircraft
But the beautiful thing with a hybrid is that you could take off on battery power, climb to your altitude and throttle back to economy cruise (which on an electric is probably below 50%). Now you start the APU. This ensures that the APU will be running at a higher altitude and consume less fuel. And all it needs to power is the economy cruise. It will also reduce noise considerably for take off. As you descend you windmill and generate electricity to top batteries up a bit. And should you really need to get down fast, you just regen more and make the prop/fan work as a speed brake.
But sure, there are still obstacles to be overcome for a pure battery powered tourer. But they're closer than many think. Battery capacity has doubled in just 5 years.
I've researched this quite extensively and what's important is power density, i.e. Wh/Kg. Newest LiPo batteries (like the R/C guys use) deliver about 400Wh/kg today. That means you could run a motor at full power of 400W for one hour. Or to put it in perspective of a C152 - a 182kg battery pack could make you cruise at 100% for one hour. In reality, you would never use 100% all the time on electric motors, so the endurance would probably be closer to 2 hrs for that weight. Not that far off - remember, you'd save not only on the weight of the fuel and the engine and that's gotta be an easy 150kg alone.
Now, here's the interesting thing - nanowire batteries that have just been patented and are getting geared up for production have a potential power density of 4500Wh/kg. If they can deliver on that promise, then it's all over for the combustion engine. Bye bye. Gone.
Obviously cost is also a factor for batteries, and to keep them healthy no more than 1000 charges are recommended. At the price of batteries today that would be a large sum of money, probably the equivalent to a TBO overhaul of your Lycoming. But the nanowire promise has the added benefit of dramatically reducing the price as well.
It's closer than we think. I wouldn't want to be Rolls Royce, General Electric, Pratt & Whitney, Lycoming or Continental in 20 years time.