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.
3DS had good legs due to decent performance in all 3 regions. The Switch will blast 3DS in the Americas but fall behind in Japan. Let's compare Switch and 3DS, equal quarters.
Switch is 14.01m America, 8.23m Japan, and 12.50m Others. 34.74m Total
3DS was 10.26m America (38.7% of Current sales), 11.54m Japan (45.8%), and 9.29m (39.5%) Others. 31.09m (41.4%) Total.
Switch has a 3.65m lead. 3DS only shipped 12m next FY, while Nintendo predicts 18m for Switch. Clearly Switch will widen the lead.
As you can see at this point in its life (March 31, 2013), the 3DS has sold ~41.4% of its total sales. Its highest proportion of sales was due to frontloaded sales in Japan at 45.8%. I think Japan sales are usually frontloaded though, with the Switch lagging far behind in Japan but far ahead in America and Others. I think we could make a guess and assume some of the % that the Switch has sold so far of its LTD.
For example if we go with 37/40/39. Then we get 37.9m America, 20.5m Japan, and 32.05m Others. Giving 90.45m Total sales. Personally, I think that is a bit strong. At 90.45m it outsells the 3DS by 14m.