> You've been able to fly a plane with unleaded fuel for decades. See every major airline burning jet fuel.

Jet engines are completely different from the piston engines you'll find in general aviation (sans choppers, which mostly run on jet engines).

Piston engines need leaded fuel for lubrication as well as knock resistance, and deviating from the original certification of the engine/plane requires a type-specific certification - every model of plane you want to fly that was certified with 100LL AvGas needs to be separately tested if it can fly safely with lead-free AvGas [1].

[1] https://www.flightglobal.com/engines/faa-approves-100-octane...