Although this means that none of us may actually exist, there is an upside: Brian Leiter may not exist either! "The problem of evil," to quote the possibly non-existent Leiter, is "solved at last." God, I understand, is pleased with the solution. Nietzsche, however, could not be reached for comment.
Poor philosophers. They rely only upon Nietzsche (who does not actually support Beckwith's position, at least not as a 19th-century reader of German would have understood the argument, but that just validates Leiter's suspicions, and Professor Solum's indirect rejoinder) and others who don't try to understand the implications of their musings as applied to people and their behavior. They haven't bothered to read enough science fiction.
I'm dead serious.
The concept of "alternate histories" has been extensively explored in science fiction, often to far greater depth than in formal philosophy, for a very simple reason: Formal philosophy fails to connect from the abstract and theoretical to the personal, whereas good fiction does precisely that. Frankly, some of the science fiction writers who have explored the topic are/were probably smarter and more rigorous in their thinking than some of the faux-intellectual "philosophers" who have pontificated on the matter; many of the latter display little familiarity with Heisenberg's conjecture or its implications. Consider the careful constructions of Philip K. Dick's The Man in the High Castle, L. Sprague de Camp's Lest Darkness Fall, Harry Turtledove's alternate histories of Rome and of the American Civil War, and even lower-brow fare like Eric Flint's alternate Thirty Years' War. Sometimes serious academic historians give it a try (although seldom with that much success as fiction).
Beckwith ignores the "law of unintended consequences." Perhaps Brian Leiter does not "exist"—or, as Professor Solum points out, is not the "same person"—only because Beckwith and his ilk do not either. Alternate histories and universes are not causation-free, by any means. And, if not Leiter, does that mean that nobody will step in to fill Leiter's rhetorical role in debate over Idiosyncratic Idiotic Intransigient Ineffable Inscrutable Intelligent Design/creationism (for, as Robert Pennock points out, they are merely rhetorical reframings of the same core hypotheses)? I think not. Perhaps Beckwith "himself" would be filling Leiter's role…