BackPacker:
Good point regarding the electric motors themselves, but also regarding the batteries! And once you have the motor and the batteries, the hardest part remains: Systemization and system integration, taking that engine and those batteries along with all the other parts that will be needed and turning it into a practical, economical, reliable (both in terms of safety and dispatchability) and generally well functioning power source. Add certification standard development, certification (both aircraft and pilots; what differences training will be needed?), and so on and so forth.
Those are not reasons why this isn't a good idea, but they are reasons for why it will take a good long while and be expensive.
Again, in the experimental world it is a different matter entirely. This is exactly the type of thing the experimental category is intended for! Best of luck to them!
AdamFrisch:
Backwards and slow...? Slow, yes. Backwards, really?
There is a difference between stagnation and maturity. By the 1950s (some would say the 1930s), airframe and engine technology for light aircraft had reached sufficient maturity for designs based on that technology to be "good enough" for many purposes.
Once that stage is reached, a new design is not automatically much better than an old one. Taking into account aspects like reliability, maintainability, customer base, market positioning and so on, the older design can be better than the newer one. Or more commonly, the newer design is better, but not so much better that it really matters much in practice. When that happens, you will not see much progress in that particular field, but that is not stagnation, it is maturity. Stagnation would be when progress stops before the field is mature. Maturity allows one's efforts to be spent on other areas, in fields that are not yet mature and where progress really matters.
The progress made in GA in the last 10 - 15 years is quite tremendous IMHO, and I think it is accelerating rather than slowing down. But the results are not seen on the outside, or in performance specifications; a C172B looks deceptively similar to a glass panel C172S, from the outside. The difference is found on the panel, and in terms of the functionality provided by the systems on that panel. As in so many other fields of technology, capability growth takes place in software, not in hardware.
You may well be right that power plant technology will be an area of strong progress in a not too distant future, say the next couple decades or so. But I do not think it is fair to say that aviation in general, or GA in particular, is backwards or has been standing still in recent years. Avionics is not a mature field of engineering, and that is where most current progress is made.