its not as hard to predict because of the staggered launches, the hardware numbers will maintain a fairly high peak up to, and beyond japanese launch because demand has outstripped supply, with many people preordering not getting their units till late december/early jan, at the same time theres a barrage of decent games from jan to august, without adding any other unannounced stuff to it.
WiiU relies on first party, that support isnt there. Xbox One and PS4 live off of both first and third party, so its much harder to experience a cold stretch.
That isnt to say it will sell astronomically forever, but the launches alone are enough to create an install base for these platforms that outstrips the WiiU's, in a much shorter time, and the costumers of these platforms have, and will continue to purchase games even if they are third party, which nintendo owners have, with little diversion from the rule, shown they will not do.
If nintendo had the manpower to deliver blow after blow, hit after hit, or had third party support and third parties had the support of paying customers, we wouldnt be having this discussion.