It's entirely possible that things don't get better (or continue to get worse) for reasons unrelated to lead. The addition of the internet is way too big of a confounding factor to say that lead was actually good though, IMO it's more likely that we haven't learned how to parent children in the modern world yet (though arguably that has always been true).

And "cognitively [un]developed" is a weird way to say it; I wouldn't call that a "bland" term at all.