From what I have heard, Ladd never explained the existence of unregenerate humans in his millennial view, he just assumed it was true. I am not arguing for or against Historic Premill (although I am Amill). I am just asking how they logically get unregenerate people into thier millennium. Still havent heard an answer. But alas, Jacob is the only one answering for them here on the PB and I am reluctant to belabor the point. I have spent some time searching online to no avail.
My suspicion is that Ladd didn't really care. While he became the representative of Historic Premil, most premils thought he was just another amillennialist (and most of his exegesis tends to go that route).
In my classes with Ladd and reading of his books, I would agree that he did not care about the question. Ladd was premil because . . . 1. He had a premil background in fundamentalism prior to his seminary and Harvard days, and 2. He did not know how to accommodate Rev. 20 into his eschatology otherwise.
Exegetically, it is almost as if Ladd was an inconsistent amillennialist who took Rev. 20 more literally. That makes him an inconsistent amillennialist or an inconsistent historic premillennialialist.