It really all depends on how well Mario Kart 8 does. Not just launch week, but for the remainder of the year. If MK 8 is a runaway success the status quo will be broken and with heavier advertising it'd only get better. I forsee a SM3DW/MK8 Wii U bundle later this year to launch alongside(ish) Smash Bros U.

If MK8 sells just modestly, then it very well will be

The release dates of MKDS and MK7 weren't all that divergent.  MK7 had a week more in Japan, and a week less in Europe.  It was just North America where the DS version had a decent amount more time (20 days).  The 3DS actually had sold more in the US at the point of Mario Kart release than the DS.

MKDS didn't just have better legs, it has completely closed the gap and is now surpassing MK7 aligned.  So selling to so many customers can't be the only reason when MKDS had sold to more customers at the same point in its life.  Digital sales also aren't the reason.  Nintendo has MK7 shipments+digital sales at 9.62m as of March.  VGC has retail sell-through at 9.4m in the same time period.

Being the cheapest console of the generation doesn't really matter if the last generation is still getting those games.  The new skylanders is still on the Wii and 360.  Disney Infinity Marvel will be on 360.  Just Dance will likely get a last gen release as well this year.

I'm also not sure if its sales lead is indicative of much with those games.  Infinity didn't get a PS4/XBO version.

Skylanders was
Wii U - 258k
PS4 - 144k
XOne - 137k

Just Dance was
Wii U - 271k
PS4 - 180k
XOne - 219k

Not exactly a monstrous lead especially considering the Wii U had a Skylanders bundle, got the games earlier, and had the larger install base (and still leads the XOne).  In 2014 sales the PS4 and XOne are leading on Just Dance, and are quite close on Skylanders.  Plus, the Just Dance market is eroding quickly, and I have to imagine the buy new game, toys and portals every year market will be getting fatigued fairly soon.

So what exactly is the breakdown of this 6 million per year?  Is it just a steady sales rate?  Because right now it is looking like the Wii U will have a tough time hitting 10m by the end of this year.  That would require it to sell through the end of 2019, three full years after what you expect is their last big year of support.  2010 was arguably the last big year of support for the Wii.  It sold 17.3m.  Its momentum carried it to less than 5.1m two years later.  I'm not sure why you expect momentum to carry a Nintendo system after support dries up.  Both the Gamecube and N64 shipped less than 750k the fiscal year their successors came out.  The DS dropped very quickly as well.

Going by your timeline of support, the Wii U would probably need to be at 15m this year and 25m by the end of 2015 to have a chance at 40m.

30 million lifetime, due to a pricecut to $250 USD in time for these holidays, a dark, epic Zelda with great graphics to win back the "hardcore" (in conjunction with Monolith Soft's X) and a technically advanced and critically acclaimed new IP/Metroid from Retro.

I'm not getting my hopes up though, Nintendo have a habit of screwing things up these days.

Honestly, Im struggling to see it ever passing the Gamecube at this point.

I'm going to try thinking about it a different way than I normally do. Here's a rough estimate of the percent chance I think WiiU has of passing these benchmarks by the end of 2017.

30 - 1%
28 - 3%
26 - 8%
24 - 35%
22 - 42%
20 - 55%
18 -82%
16 -97%

Three years after the PS3's launch it had managed to sell just under 8 million units.

How are you so... wrong?

PS3 had sold 27 million units by about 3 years into its life, mid Nov. 2009.

I think that at this point Nintendo doesn't really care how much they'll sell. They are back to their roots and probably feel a bit comfortable again - they no longer lose money on hardware and will get a decent boost in the future. I think the fact that there was no price cut shows us that they don't want to fight for sales, but rather are interested in getting as much money as they can in the safest way possible. I think this is smart, Sony has been doing this with Vita for quite a long time until the production costs went down significantly. It's starvation mode, but they can't do anything else.

Profit on hardware + good sales of first party games is their target now, they don't really care about any given number of Wii Us sold. I think they'll cut the price the moment the production costs allow them to still be profitable when selling the console for $50 less than now. It's sad for the gamers, but wise for Nintendo.

Still, I hope that both Wii U and Vita will gain more momentum.

Best case scenario is 20m. I'm expecting 15m.

Probably...

Many console who struggle in their early years but become success at the end of their life like SNES and PS3.