They don’t “know” that lead is “needed”, and didn’t “know” that when they first added lead to avgas. What they did know back then was that lead was a pretty bad substance to use as an octane booster, but it was a quick-and-dirty solution that happened to work. I doubt any of them thought lead would be as long-term a solution as it ended up being.
I don’t buy the argument that the FAA was pressured to accept GAMI’s STC either. At least, not from outside forces. They don’t really have anything to gain through its approval. If they were truly forced to accept it, I would have expected Congress to be the one forcing them to approve it (as they did with BasicMed).