Well, it's very unlikely that the biggest world leaders would be usurped by demons because regardless of demons merging indiscriminately with humans, it's established that unless someone has completely lost their reason a merger won't be successful. Demons would more than likely be able to infiltrate our ranks to some extent, but they'd probably lose a lot of their numbers doing so and I doubt they'd be able to destroy all seven billion of us without having access to nuke buttons. If Satan and Xenon take a more active role in wiping us out though, the outcome may turn out the same way it did in Devilman.
