If Smash and Mario Kart weren't the massive system sellers people expected them to be, neither will Splatoon. Will there be people that buy the console just to experience the game? Definitely, but I doubt it'll pull an adoption rate as high as Smash did when it was released. I can definitely see Splatoon being a mainstay IP for Nintendo, with one in every console like other heavy hitters. It's hard to admit, but I don't think any single game can "turn" the Wii U around at this point. I frankly could care less though, so long as the game is fun and the community stays active - which it is - Splatoon should have a nice lifespan and bring tons of new people on board.