Every. Single. Generation. ZOMG WE'RE GETTING A LIGHT TYPE THIS TIME. Last time it was because of Reshiram, and this time Xerneas is the culprit. Seriously, just... stop. Steel and Dark really only exist because gen 1 was so incredibly imbalanced, and while some types are still better than others, adding a new type isn't exactly what's needed to change this. Not to mention, introducing a new type now will make it incredibly underpopulated compared to the currently existing ones. A problematic that wasn't much of an issue in gen 2, if you understand basic math.