Neoxon is talking some sense here.
Let's look at this Wonderful 101 and Pikmin 3 comparison, since these are the parallels most folks want to draw when speculating on Splatoon's sales. You've got a new IP making it's debut on the Wii U alongside a mid-tier Nintendo IP, if we're comfortable with that term (far from Mario and just as far from Dillon, let's say).
As has been noted, Nintendo put very little effort behind marketing TW101, and it had a game on its hands that -- love it as I do -- was incredibly inaccessible and faced middle-of-the-road reviews. Similarly, the late Summer of 2013, when both games released, was an absolutely abysmal period for Wii U sales.
By September 2013, the Wii U had an install base of 3.91 million units. Anecdotally, the console was treated like a cancer by gamers and press. According to a site that shall not be named, just to give us a rough idea, TW101 has pulled down a little over 300,000 copies while Pikmin three has done just under 1 million.
Now, let's look at Splatoon's situation. The marketing seems to be there. From Miyamoto talking the game up to the current art contest, the Tumblr, the recent (and glowingly positive) press hands-on, the "Mess Fest" going down in L.A. this month -- in any case, Splatoon has enjoyed a
hell of a lot more Nintendo-generated buzz than TW101 (or than Code Name S.T.E.A.M., another common comparison), and it's still about a month from release. The amiibo factor also can't be underestimated; I don't think I need a citation to illustrate how insanely popular they are. More than that, you have a
significantly more positive Wii U landscape. This is the post-Mario Kart, post-Smash, post-amiibo Wii U. It ain't setting the world on fire, but its mindshare seems more positive all-around, and its install base was up to
9.2 million by February 2015 (giving Splatoon another four months of install base growth to take advantage of -- I'll edit if anyone wants to dig up more current numbers).
I don't know how you can look at that and honestly think that Splatoon will do Wonderful 101 numbers. I think 1 million is a fair estimate, given the marketing, amiibo factor and far larger install base.
Will it be the next "major" IP? No, it won't be Mario. It won't be Zelda. But I predict it'll likely be an evergreen, slow-burn title, like many of Nintendo's games historically are. Nintendo is also cognizant of Wii U sales figures; we can safely assume the company doesn't have Wii-like expectations for the sales of their Wiii U games, and that they temper the budgets accordingly (I think they also view the games as investments for the Wii U, as their philosophy is that good games sell consoles -- thank the fuck Christ).
That said, if it does solidly on Wii U (again, around 1 million), I think we'll see a sequel on a new platform that has, theoretically, a much larger install base.
TLDR: Yeah, about like Pikmin.
