3.6 million is probably a spot on estimate.
For all the hoopla, Nintendo really only has 2 total games that would impact hardware sales in any significant way -- Mario Kart 8 and Smash Brothers.
As much I'd love to be wrong, if Donkey Kong Country, a perrenial 5+ million selling IP can't even move the needle on Wii U hardware sales, things like Bayonetta 2 and X are not going to either (lets be real). Lets not even start with the third party support, that's just a disaster.
And if you objectively look at it, Smash Brothers historically sells about the same as Mario 3D, so there's not much of a reason to assume Smash U will cause a bigger bump than Mario 3D World did last year.
That leaves it to Mario Kart to basically push sales about 900k higher than last year ... which is reasonable. The rest of the lineup is fairly comparable to last year, Smash = Mario 3D World, Hyrule Warriors = Zelda WWHD, X/Bayonetta 2 = W101/Pikmin 3, Yarn Yoshi (early 2015?) = DKC? Honestly last year's lineup may have the edge in a few of those, but Mario Kart is the difference.
Also factoring in that X1 and PS4 will likely be stronger this fall (possibility of further price reductions and better game library) and continued erosion of the casual audience to tablet/phone gaming ... yeah 3.6 mill sounds about right.
