Kind of needs a caveat: ... for engines not designed to safely operate with other fuels. I.e. it's not that we need Avgas 100LL to make piston-engined planes possible, we just have a lot of plane engines around that need it. (and one can argue that industry has sat on the problem a bit, because why change as long as 100LL is readily available. (and making 100% compatible fuel replacements indeed appears to be difficult, as far as I know there is only one that has any kind of permits for one plane type, and that also has somewhat nasty additives - so changing the engines is the thing to do)