I think it will.
It's got a strong fan base behind it now and everyone knows what to expect. This is one of the games you buy a Switch for. And with the bundle it has in Japan and if that bundle is confirmed for everywhere else, I think it is very likely that it will do it.
The first game managed to pull off close to 5 million on the Wii U as a new IP. And it managed to help the Wii U beat the PS4 in Japan for the month if June 2015. Now that it's going to be releasing on a system that the mainstream public actually wants and cares about, the sky is the limit for Splatoon 2.
I believe this so much that, in fact, I'm going to take this bold prediction and make it even bolder.
Not only will it outsell the first game in it's first year, it will do it by a very comfortable margin. AND it will be the first game for the Switch to reach 10 million. AND this game will even reach 15 million and stand on the same pevel as Mario Kart, 2D and 3D Mario, and mainline Pokemon in terms of sales. Nintendo is about to get an All star starting lineup that could dismatnle the Cleveland Cavaliers and Golden State Warriors, you heard it here first!
Mario, Pokemon, The Legend of Zelda, Animal Crossing, and now Splatoon!
...So this is what it's like to be Tbone.