You can argue for its convenience/speed/fun, but fuel efficiency does not look good for GA aircraft. Even for your modern GA aircraft example, it literally comes to 16 mpg. This is as bad as worst of the trucks out there. While it's great if existing use cases of GA aircrafts would become more efficient, adding demand in this area (more hobbyists, more rich people shuttling use, encouraging people to live in very remote areas, etc.) and using it as transportation-mode will unlikely ever become environmentally reasonable.