10-mana cards are nearly universally unplayable. Only Deathwing, which literally clears the board and then gives you a 12/12, has ever been played, and then only in extreme niche situations like one rare variation of control warrior. C'Thun will be played because everyone will want to make those decks, but they'll never see tournament play or any consistent high-level ladder presence. You cannot make a deck out of uniformly bad minions that you would never play based on their stats (with like one or two potential exceptions like the cheap one with the 2-damage battlecry) and then a single 10-drop legendary that comprises the entire foundation of the deck which then completely falls apart if you don't draw C'Thun in time to eke out a shaky turn 10 win after a game of playing vanilla-statted minions with no deathrattles or any other redeeming features aside from maybe the druid one with taunt. There's no rational reason why such a deck should be viable.
