@OP:
Right now Wii U is still widening its lead, not only Global vs. just USA, but also Global vs. Global.
But who knows, I can agree with you that with the right games XBOne sales will grow, particularly in USA and UK, but I wouldn't rule out he possibility for Wii U to grow too, maybe less than XBOne in US, but more elsewhere. So they could eventually almost tie.

I still dont get why people use game sales on 2013 as an example. The Wii U is in such a different position now.

XOne will be like The X360 in Japan. Japan is Nintendo's with about two million consoles sold. Wii U is winning Europe behind the PS4. XOne numbers been on a decline in the US. Look at the big picture.

I don't think the Wii U is going to sell much beyond 19 million LTD worldwide.

Can MS sell more than 19 million in North America? It will be close.

Right not I don't think devs have really shifted to next-gen development, there's still too many multi-gen titles ... but starting this fall I think the switch over will start to accelerate. That will benefit the PS4 of course, but the X1 will also get a boost from it.

There's not much overlap in audience anyway though ... Wii U is pretty much strictly a Nintendo box for the Nintendo fan from here on out. XBox is more of your all purpose/wide variety of genres type of primary console. 

Short answer: I think anything could happen at this point regarding XB1 vs WiiU. Logically, one would assume that XB1 would win because the audience for 3rd party games is bigger than diehard Nintendo fans. Throw in a big name exclusive like Halo and it further strengthens that point of view. And if it were just Microsoft's console on the market against Nintendo's, then yes, they'd have their win on lock without a doubt.

Problem is, XB1's biggest advantage over WiiU (3rd party games) is handily negated by the presence of the Playstation 4, which is poised to be the go-to place for the majority of those consumers. No matter how you slice it, Sony is first and foremost Microsoft's direct competition... and the PS4 is going to win that battle.

Meanwhile, Nintendo's entire game library is completely different and offers unique experiences from the competition, which gives it good potential as a second system, plus the fact that WiiU will remain the cheapest console. Then, what if something like Mario Maker proves to be pretty popular WW and helps with mass market appeal? What if amiibo is a breakout success? What if Zelda U becomes the next TLOU with the critics? There are a lot of variables at this point.

If I had to guess, I'd go with XB1 finishing in second place if for no other reason because I think it'll be around longer before its successor comes along, but in any case, things will be much more clear about a year from now.

lol take a look at sales, clearly they do.

Thyere no logic to x1 selliong well enough in US to out sale wiiu ww, PS4 always trumps any pro reason X1 you can dream up. 

They really would if they werent on the far more successful PS4.  With them selves id say.

People in my circles see the XB1 as an "also ran", with little to no reason to get a XB1 over a PS4, and see the WiiU as the "2nd console" for Nintendo exclusives.

Not only do I think the OP prediction will fail, but the WiiU will outsell the XB1 this gen outright. he PS4 can't be beat at this stage, and Nintendo is still making exclusives that sell.

The original XBox (2001) sold 16 million units in North America, and lets keep in mind MS pulled the plug on this system prematurely to launch the 360.

I think it's quite possible the XBox One outsells the XBox in North America, especially if it has a full five year life cycle.

The Wii U right now is trending a fair amount below the GameCube, which finished with 22 mill LTD. So no, the OP's assesment really isn't that far off.

If the X1 outsells the XBox (2001) in North America and the Wii U continues to sell below the GameCube ... it's very likely actually that the X1's North American sales will eclipse Nintendo's sales.

Nintendo is a great company, but the appeal of a Nintendo home console without a breakthrough gimmick like a Wiimote that brings in tons of new gamers is simply quite limited. MS has much wider appeal simply because they have much broader developer support, they're not dependant on just one company's games. 

People don't want a niche library console, even if some of the games are truly great ... they want a console that plays a really wide variety of games from many devs, this has even been true of successful Nintendo consoles (the NES and SNES). They had more overall content than the competetion. 

Just to play devil's advocate here, people are already getting that system you describe in the PS4.  Suddenly, Nintendo's "niche" library becomes more appealing to them as a second console than a system with 95% of the same games.  True that this didn't help the Gamecube vs Xbox, but the XB1 also doesn't have its 6th gen selling points of being more powerful than Playstation or having a vastly superior online experience anymore.