General Aviation (private planes) are STILL spewing lead into the atmosphere. It's not like they don't have the money to fix it, they just don't care about the externalities.
https://www.faa.gov/nextgen/where_we_are_now/nextgen_update/...
It seems unwise, chemically speaking, to expect we'll be able to find a drop in replacement that's less toxic (vs creating something that we find out is toxic in another 50 years).
I don't think it's out of the question that improvements in refining technology has made it possible and economical enough to create a fuel with a high enough octane rating without resorting to TEL or other metal-organic additives. E.g. the Shell stuff that is being investigated by the FAA PAFI project is rumored to be based on super-alkylate + aromatics.
Presumably you're tangential to the chemical-synthesis industry? It seems like there's a disturbingly low bar for "We found a compound that will do a thing! Okay, as long as it isn't food / a drug / incorporates already banned substances... put it into production!" (20 years pass) Substantial toxicity found.
And no, I don't work for the chemical industry, or anything related to that really. But yes, I've heard of such cases where the replacement for some banned product turns out to be even worse. I do think one should be very careful about trusting environmental statements by the chemical industry, or any industry for that matter. Just like one shouldn't trust them blindly, I don't think a blanket dismissal of anything new they are doing is warranted either.
GA planes also generally last a lot longer than cars, you have Cessnas from the 70's still flying with their original engines from the 70's.
"But they still work!" is not a good excuse to pollute.
Only the smallest private planes run on piston engines, whose owners don’t really have money to “fix it”, especially because FAA regulations for certification of new planes and engine designs make everything cost about 10x what it should.