In the long term, whether the Wii U or Xbox One grabs second place is going to rely, probably, as much on the success of the PS4 as it would the Wii U; as someone said earlier in the thread, for first-two-console purchases, a Wii U and PS4, or Xbox One and Wii U, (in whatever order pleases you,) makes a fair bit more sense than a PS4 and Xbox One, especially when Nintendo's heavier exclusives start dropping. Put simply, far less overlap and more unique content by making a Wii U your second console; the only time it doesn't make sense, obviously, is if you aren't interested in what Nintendo's library has to offer.
With that in mind, the bigger a piece of the pie PS4 carves for itself in the various markets, the more likely it is that the Xbox One's piece will get smaller and smaller as it loses potential customers who simply won't see the point in getting both. Nintendo, because of both the aforementioned lack of library overlap, and its cheaper price, is more likely to be picked up as a second console than either the PS4 or Xbox One, and since both Playstation and Xbox users would be doing this...
So, to summarize; if the Xbox One stabilizes its position against the PS4, and turns it into a much closer race, especially if it takes back the U.S., then it will be in second place.
If, on the other hand, the PS4's average sales lead grows year after year, and the Xbox One's long-term 'marathon' sales suffer as a result, then it becomes much more likely the Wii U will sneak in there.
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Or He Procrascinates, In Which Case Zanten, Doer Of The Things Later
Or It Involves Moving Furniture, in Which Case Zanten, F*** You.
