I agree that electric propulsion is the future of transportation, after all, dino juice can't last forever...
There is a third way: biofuels. About as sustainable and carbon-neutral as a battery, with about the same energy density as fossil fuels.
One possibility is
synthetic UL91, which has already been used to fuel a Rotax-912-powered Ikarus C42.
Another possibility is ethanol. (By "ethanol" I mean
pure ethanol, E85 to E100, not "gasoline with some ethanol in it" like the E15 that we buy at the gas station). It does pose some challenges - which is why engine companies like Lycoming and Rotax tell us to avoid it - but it is possible to overcome those challenges through R&D, and (after enough
testing and some
minor modifications) to run most piston airplane engines on ethanol. In Brazil, a large fraction of cars have been
running on ethanol for decades, and cropdusters are powered by
certified IO-540 engines that run on ethanol. It's not rocket science. It's just a matter of testing materials until you find reliable ones that are not corroded by ethanol, for things like fuel lines and gaskets and
fuel pumps and so on.
A third possibility is biodiesel / synthetic Jet-A, which is what most people mean when they say "
Sustainable Aviation Fuel". This is already used by
airlines and
jet manufacturers and
bizjet operators and the
USAF and
Navy. However, there are only very few people who fly piston-powered airplanes
who want diesel engines, which have the side benefit of being able to burn Jet-A. I won't go into
all the pros and cons of those engines, other than to say that their high price seems to prevent them from becoming super popular.
For example,
the Vanguard Squadron has been flying RVs on biofuels for many years.
I would not recommend that anyone start flying their airplane on ethanol, or any other alternative fuel, without first developing a thorough understanding of how each material in your fuel system holds up to years of exposure to the fuel in question, including mixtures with water, or on surfaces that is mostly exposed to air but occasionally to fuel, etc. When you start using an alternative fuel before there are reliable materials and modifications for it on the market (based on R&D and, ideally, years of experience), you're basically engineering your own fuel system.
But it's only a matter of time until we all have to choose one of these options, and are forced to do the R&D and testing and modifications required to safely use synthetic fuels (or
hydrogen) or to improve batteries, because dino juice won't last forever.