I think, from a sales perspective, it won't be the next SSB and MK. The first Mario Kart sold over 8 million and Smash sold over 5 million. I think it will be on the level of Star Fox (The SNES game sold about 3 million, I think Splatoon will do better than that, but be on the same level).
This is a terrible comparison. Splatoon has to deal with a userbase 5 times smaller than the SNES and over 3 times smaller than the N64. It also doesn't feature gaming's biggest icon, unlike the other two, of which he even has his name in one (and honestly, even SSB's name is still a reference to SMB, so...).
I'm not saying Splatoon will be as big as the other two, but it will be big, in fact it already is. You might say Nintendo does crazy stuff sometimes, but there is no chance they're killing off a multiplayer-focused, brand-new take on an existing genre (aka what they're known for) that's selling millions on a tiny userbase. And by no chance I mean no chance.