Seeing the numbers, it seems that the console sold around 3 million units in both 2013 and 2014. To surpass the 15 million barrier, it needs to have other two years selling 3 million units and in third one it needs to make 1 million. 

That said, it's possible. Next year the console will get Zelda U, this is the only "system seller" it will have in that year, so Nintendo needs to be aware of any potential that titles like Star Fox, Mario Maker and Splatoon can have to attract consumers.

Another thing it will need it's a price cut. 2013's one demonstrated many people show interest on Wii U but they still think it's overpriced.

So I don't think 2015 should be an issue, 3 million units seem kinda hard, but it's very possible.

Now, 2016. This is he hard part lol I think Nintendo will need a new marketing approach, offering every Wii U exclusive title to date to those consumers that haven't even shown any interest in the console. They should say: "you are missing many quality titles such as Zelda U, Star Fox, Mario Maker, Splatoon, Xenoblade Chronicles X, Smash Bros., Bayonetta 2, Mario Kart 8, DKC Tropical Freeze, The Wonderful 101, Pikmin 3, Super Mario 3D World..." The list goes on and on. 

We also don't know the kind of titles Nintendo will publish in 2016, but Animal Crossing should help, at least in Japan, however is hard to assume this because, of all Wii U exclusives, only Pikmin 3 has shown any power of "system seller" in Japan. 3D World, Kart 8 and Smash failed miserably, so Nintendo needs to go with caution. An exclusive Dragon Quest, Final Fantasy should help too, but you know, that's not really possible. 

Metroid could do something for the console in the west, but it should appeal to mainstream audiences, and that is another first person title like Prime is. However, again, Nintendo needs to be smart because this hypotetical Metroid title will need to have something "cooler" that other FPS such as Destiny, Call of Duty, Killzone, Titanfall, Halo, etcetera; have.

2016 sales will depend on Nintendo's approach and how it directs its own titles. It's less possible now, but it can make 3 million in that year too.

Unfortunately, I don't think the console can make something bigger than 15 million. 

Would you mind explaining to me the wonders Smash has done for the Wii U's holiday sales, please?

15 million isn't a big number because the Wii U can't sell a big number. There's absolutely no way you can actually think the Wii U will get a 50% price cut. You've got to be kidding.

Nintendo did not make money on hardware at launch before the Wii, especially not with the GC (price was slashed before launch).

Nintendo won't release a new console mid gen, 2 or 3 years before the Sony or Microsoft, that's extremely unlikely. Even if they are not direct competitors, it would be very confusing for their product. The new handheld is more likely to release in 2016.

Wii U will 100% make it to 15M+. I mean it hasn't even hit $249, let alone $199. It also still has a few good exclusives that haven't come out, mainly Zelda and a future Mario game. Of course, I do think it will struggle to hit 25M. My guess is it will wind up around 20M-22M. Well, unless sales go nuts once it hits $199, but I don't really expect that to happen.

The issue is that that is not the only problem. 

I'll give you one example of another problem, which I think is a much bigger one: A few days ago one friend of mine who is kind of a casual gamer (well, he loves games but he is poor (just like me) so he cannot afford to go all hardcore like most people on this forum) anyway, he was talking to me about the Wii U and how he thought the Gamepad was "really cool", and how he was considering on buying one, and asked my opinion on the matter. I told him: "well, that depends. Which games do you wanna play?". His answer was: FarCry, Call of Duty, GTA and Mortal Kombat. So I told him: "well, you won't be able play those games on the Wii U". He couldn't believe me until I showed him some websites and then he went on to say that he would decide for the PS3 instead.

So, to summarize, I think that not having all those popular games on the Wii U is hurting the system a lot more than Nintendo (and its fanbase) realize.

We will wait getting full results of the end of this year to judge Smash.

But 150 is a price the Wii U will reach, as the GC reached 99 and so did the Wii. Will it happpen at the end of next year or in 2016 is the real question. The extra 50 is the price of the gamepad that can't be reduced.

What? Where did you get that from? The GCN was profitable its entire run, from what I've read.

Both will in 2016. The only way it will be "confusing" is if they fuck up the names like they did with the Wii U. Since they will be most likely branded together, as opposed to being branded with their predesessors, they'd need to try mighty hard to fuck it up. 

It might not hit 15. But to say it can't is silly. We haven't even seen how it moves at a bargain price yet.

Ah, of course, getting of the Gamepad. I'm not in favor of this idea, but it should help making the console cheaper adn making it more attractive than Xbox One and PS4.

Nintendo can't stomach a dual launch the same year.

The Game cube was not sold at profit , but Nintendo was making money on games. Nintendo explained the price of the Wii (gamecube rebooted) by saying that contrary to what they were doing normally, they wanted to profit on hardware too.