We also know that 100LL is shelf-stable for years and is compatible and safe (from the perspective of those riding in the plane) to use with just about every piston aircraft engine in existence. We don't yet have enough data to know if that's the case for the potential replacements. Automotive gasoline is too low-octane and not stable enough to be used most aircraft, which for some aircraft results in engine failure in flight. There are ongoing efforts to develop an alternative aviation fuel blend, but as I mentioned previously - this takes time.
A couple years ago we got close to having an approved alternative - Swift Fuel's UL94. However as an example for why this is hard: one of the objections to that fuel was that it didn't weigh the same as 100LL, meaning its use would alter weight and balance designs of the aircraft that use it. For most airplanes that's not a big deal, for some it is.
We have 80 years of aircraft flying around designed more or less around the characteristics of one specific fuel blend and most of those aircraft have decades of life left in them. We all want to get off 100LL, but we also don't want aircraft falling out of the sky as a result.
But in aviation you're not only worried about the knowns, but also the unknowns. Example: JAL 123, BA 38, TWA 800 and other cases where you believed things to be safe but in reality they were a ticking time bomb and/or would trigger in very weird conditions.
