If comparing tracking is your argument, why would it pass 100 million, if it's tracking below the Wii? Significantly below, too, since Wii did almost 26m in 2009 FY. Unless you mean the Wii was sabotaged somehow, which is eh...
Besides, the 3DS was an unique slow burner as far as modern Nintendo hardware goes. It sold more than a third of its lifetime sales after four fiscal years in the market. Don't count with the egg before it hatches etc.
3DS was very frontloaded, by end of 2014, it was already at 49M, which is already 2/3 of the total sales. What happened afterwards, was that during 2015-2018, sales didn't really budge, and the 3DS sold about 7M a year in that period. But in those 4 years, it just about equaled the 2012-2013 sales. And that's with numerous hardware updated in that period (2DS, New 3DS XL, 2DS XL), so those 7M a year are nothing to write home about.
The reason why the 3DS was selling about the same over the years was just lack of competition, both from Sony and from a successor. 3DS was 7 years on center stage of handhelds, and until now if you consider the Switch not portable enough. And since Sony doesn't seem to give the Vita a successor, chances are that the Switch will be in a similar position, whoever wants gaming on the go pretty much has to choose the Switch until Nintendo brings a successor. In other words, the Switch also will have a long sales tail, and not a sales crash like the Wii.
Switch might not be able to fully keep up with the Wii, but is already largely distancing the 3DS and will sell longer than the Wii; the Wii's advantage is in it's last growth year, starting next year Wii sales will start to drop, and the Switch will be eating up the Wii's advantage over the course of 2-3 years.
The Nintendo eShop rating Thread: http://gamrconnect.vgchartz.com/thread.php?id=237454