> Lead gas would eventually reach the point of doing so much damage that people wouldn't want to use it anymore, even if it was cheaper. Even if it didn't get to this point, eventually it would be killing/debilitating so many people that it phases itself out.

For that to be the case:

(a) you'd have to be able to make the causal link between societal damage and lead from vehicle fuel, which isn't something most people are capable of doing as individuals

(b) you'd have to have a choice in the matter as an individual consumer, which you didn't have for cars, because automakers produced engines that (they said) required lead gasoline, and the situation is similar for AV fuel