So, in Europe, take out the reliance on avgas and what have you got? A bet on future tax policy...
The tax situation is different in each EU country. However, I don't think it is a major factor. Avgas is getting rare (outside Northern Europe) and getting more and more expensive due to the very small production volume and separate infrastructure. An unleaded replacement would suffer from the same economics of scale issue in Europe (I bet a busy gas station in the UK sells more petrol a day than all airports in the country sell avgas on an average day). What makes things even worse, the avgas burning portion of GA keeps shrinking in Europe. The Rotax powered fleet tend to use 98 octane mogas. Unfortunately the EU now requires the oil companies to add up to 10% ethanol which the Rotax engines/fuel systems are not certified for so aerodromes have to determine the ethanol concentration for each fuel delivery.
I don't see any real alternative to diesel piston engines outside of the US. I read that the companies that built the skyscrapers in Manhattan all went bust. The ones that bought the towers at a bargain price became very rich.