Serious question: what's wrong with lead in avgas?
No one serious would ask what was wrong with tetraethyllead in gasoline fueling automobiles in urban areas, but aviation is a completely different context.
It's completely unnecessary, as it has been shown that general aviation planes can operate on mogas in almost every single propeller engine. Additionally, it costs more over the long run due to the maintenance costs (e.g. spark plug issues from lead build up when you sit at lower RPMs for taxiing).
Lead isn't added to help increase the lifetime of anything. In-fact, that's hilarious, because lead fouls spark plugs and the rest of the engine. I've personally removed spark plugs with pea sized nuggets of lead wedged in the electrode. Problems caused by lead (e.g., stuck valves) are probably the #1 reason you have to overhaul cylinders. Lead is an octane booster, that's it.
"Knocking" is an antiquated term. Knocking = detonation, and any severe detonation would destroy an aircraft engine. For example, this is why we take off with our mixture so rich, to create a huge margin against detonation by lowering internal cylinder pressures.
> > 70% of 100LL aviation fuel is used by the 30% of the aircraft in the general aviation fleet that cannot use any of the existing alternatives
A large part of this problem, AFAIU, is that "cannot use" can mean lack of type certification. IOW, as a practical matter many (most?) pre-existing engines could use new, unleaded fuels, but engine certifications haven't yet been updated by the FAA. Only leaded-fuel is legally permitted in most old engine types, and old engines dominate in general aviation.
See, for example, this blurb further down in that Wikipedia article,
> In 2022, Paul Bertorelli of AVweb reported that the FAA is dragging its feet on broadly certifying G100UL, delaying approval of the fuel for more engines and spending over $80 million on EAGLE to re-start a search for an unleaded fuel when G100UL has been under evaluation for over 10 years.
where G100UL is (I think) the only alternative replacement fuel actually approved so far, certified just last year, and only for one particular family of engines.
No one serious would ask what was wrong with tetraethyllead in gasoline fueling automobiles in urban areas, but aviation is a completely different context.