I would just like to chime in to say that I think 2019 and 2020 will be huge years for Nintendo in Japan – bigger than what we have seen in 2017 or 2018.
Likely this year, Nintendo will move to offer a cheaper and more scalable (manufacturing cost/MSRP) alternative to its hardware. As in DS Lite.
Pokémon and Animal Crossing (and Yokai Watch + Dragon Quest XI S – to lesser degrees yet still very notable) are all announced and very likely to release this year to exactly the type of demographic that Nintendo historically has targeted with lower-priced hardware.
Young Switch owners will also be able to tap into a large back catalogue of high quality evergreen titles, which will increase the value proposition of the Switch.
The combination of being able to produce a larger amount of hardware units at a more affordable price to the company and its consumers, along with having the exact catalogue of titles that have vast appeal to the Game Boy/DS demographic, could lead to the most exciting year for the Switch yet.
We are also long overdue for a new Monster Hunter Portable, which has historically been a multimillion seller on Nintendo platforms.
As a nice bonus, it will have been 2 - 3 years since Koizumi san (Mario Odyssey producer), Konno san (Mario Kart producer), and Fujibayashi san (Zelda BoTW director) have been credited in major projects. Nogami san (Splatoon 2 producer) also has not been active other than overseeing Splatoon 2 DLC and content updates. It is highly likely that these names will be associated with some major upcoming titles – some of which could be announced this year.
I don't think we will again see DS numbers, but I do think Switch will reach beyond that of 3DS.
I'll be happy to eat crow come January 2020 and again in January 2021.