I loved flying the 310 and the 340, but they are the preeminent example of what society and economy will not tolerate in decades to come - 100 liters per hour of leaded gas to move at most 6 people. A huge carbon footprint for a very small benefit in society's eyes. Such airplanes will never be converted to more efficient engines. Even if they came to exist, the modification costs would be prohibitive.
Perhaps like the predicted end of advanced life, the cockroaches survive. The Cubs, Cherokees, 172's and 152's have a chance of surviving the decline of GA, as he more advanced aircraft succumb. The modest singles can be maintained with economy, and enough will remain, that re-engining them is remotely conceivable.
And, we'll see some growth in very austere two place aircraft. New, and seemingly advanced (lots of electronics, few old style systems), but they will come with more limited capability in one way or another.
Imagine the awe in seeing a genuine Spitfire or Mustang being expressed simply at the sight of an original Cub, or 172 arriving. That could be the GA world in decades to come....