there was excellent evidence against leaded gasoline even before it was commercialized. lead had been known to be toxic for two thousand years, many of the workers in the pilot tetraethyl lead plant died, and midgley (the inventor) gradually became paralyzed, probably as a result of his exposure. people used leaded gasoline to kill insects, and accidental human deaths from ingestion were fairly common

there are in fact cases where people used something harmful for a long time before there was good evidence that it was harmful (examples include kohl, sassafras, trans fats, maybe asbestos, radium, twitter, and facebook) but leaded gasoline is not one of them

there are very many more cases where people used something for a long time before there was good evidence about whether it was harmful, and it wasn't. almost everything ever invented falls under this rubric

conservatism like yours was the default stance for thousands of years, which is why human life was impoverished and short during that time. during the 19th and 20th centuries, we took a break from it, and the result was a leap in human prosperity to previously unimaginable heights. now people want to bring it back. i'm not a fan