A purely rational person would be nigh omniscient

I agree with the rest of your point, but I think I'm misinterpreting this statement, because it seems like an overstatement to me. That is, I'd restate your second sentence as "If there was dispute over whether use of a combustion engine did more good than bad, a purely rational person would be able to effectively investigate and correctly determine the answer." As you say, I'm fairly certain that the combustion engine created more benefit than than harm to humanity.