While I, personally, don't subscribe to that theory, it's undeniable that the goblins have to be relevant at some point and unless they sweep in like a plague near the end to try and cuck the cuckers... I don't really see where they could be added. They're too much of a chekhov's gun at this stage and the villains actually being goblins would be a way to handle that, but it would also fly in the face of the savage rutting beasts that goblins have been portrayed as so far. Really, though, I kind of miss when the men were so ugly that they were a different art style instead of just being ugly, at this point Ryuu's grandpa isn't really ugly at all so much as he is just old.