There’s likely a bit more to it than just the “ancient rivalry,” there’s probably some more specific reason that Kong gets involved in Godzilla’s reign of destruction. But once that happens, the ancient feelings probably kick in. Not that we necessarily need that much of a reason for Godzilla and Kong to throw down. In the end, for the viewers, the spectacle of the battle is an end unto itself. Although, the new trailer also does explain where all these pesky humans come from, as there’s basically a group of humans following each of the Kaiju, trying to protect them as well as understand them.