I guess they are betting on c'thun decks countering it. Or more to the point, they are betting on people spending a few months trying to counter it. If it doesn't work out, they can always nerf again.
C'Thun Priest probably counters it almost just as hard as Control Warrior does. After Alexstrasza plops down they can play a single Twilight Darkmender and kill Alex and go "lol try and kill me now".