It's just that the game's they pushed during the early life cycle of the WII U had a more unconventional style to them. You didn't see them mass-advertising COD or any game that is considered truly conventional but they usually advertised third party games which made a lot of use of the gamepad and there own games such as NintendoLand which are far from conventional.
Now let's say that we also agree that the Nes was an unconventional console. How does that lead us to predicting the possible future popularity of the NX.
With the NES being an unconventional console, the Wii ceases to be an outlier. A pattern emerges where Nintendo sees decline when they adopt console wars mentality, but sees success when their primary motivation isn't to beat someone else at their game. The same pattern also holds true for Nintendo handhelds.
Taking the above into consideration, if NX was going to be a console that is about doing the same things that Sony and Microsoft do, we could quite safely predict another decline. However, the few things that are known about NX point into a different direction. Offering a brandnew concept suggests an unconventional approach, so the premise of your thread is turned on its head. If history repeats itself, NX will sell more than Wii U.