I would not bother to say anything "wise", just "bye, it was fun while it lasted".
What impact could my words have on mankind?
Mankind is too stuborn. They will keep doing what they are doing, right or wrong. Even if you tell them what they are doing is wrong, they wont change anyway, unless something happens that personally affects them.
People in general don't learn from other peoples mistakes, unless they have been personally affected by the mistake.