I am sure the same was said about Diesel cars in the past, and if you look at the USA ,in California you can't even buy a Diesel VW which does 60 MPG , yet you can buy a crappy new petrol Mustang which does 20 at a push (and has a 3 speed gearbox). Every second car in Europe is a diesel, it is efficient, and is the norm (and arguably better), and the technical advances are light years ahead of the old skool US cars with diesel engines running half a million miles without batting an eyelid.
I agree but the problem is the aviation market isn't big enough to spread the development costs of new engines and make a product that everyone can afford. Lycoming and Continental had the luxury of living through the hey days of GA aircraft production in order to develop and refine their Avgas engines. Those days have gone, there were more aircraft sold in a few weeks then than there is in a whole year now.
I agree the diesel is where we should be heading however unless there is development that can be plucked plucked from a mass market (probably automobile) without any or much further development I don't see much new in the future for GA aircraft engines. Remember also where the biggest market is for GA aircraft, and that market right now or in the foreseeable future doesn't have much of a demand for diesel aircraft engines.
Another example, the flash new EFIS screens with their AHRS that we are now seeing in GA aircraft, like the Garmin, Avidyne, Aspen etc owe their heritage to the stability systems used in the modern car. If is wasn't for the mass production in the automobile industry to spread the development costs over we wouldn't seeing these nice glass GA cockpits.