In fact, I think that in the time of the Shadow Isles' inception, the world wasn't in balance. Thinking about the way that the lore plays out, petty squabbles amongst nations aside, there wasn't a compelling negative force to contend with. Previously, it had been Mordekaiser, but his skull was stolen, and so there wasn't any way for him to be resurrected. Obviously, Demacia and Noxus were still at each others throats, but they were upstart and not anywhere near where they are now, but everywhere else, which I take to be either with the King, or minding their own business, was at relative peace. And then the King corrupted the Shadow Isles, and chaos spread. Over time, the city-states as we now know them emerged, places like Zaun and Piltover, and suddenly, everything was chaotic.