If it can hit 40 million by the end of 2020 ... I mean it's not exactly "lets throw a party" type numbers, but it would be a good deal enough to ensure a Switch successor, you're probably looking at a LTD finish of 50-57 million in that case.
*If* Nintendo can get a new IP that really, really breaks out (or Splatoon 2 explodes like wild fire) then I would revise up from there, but I think this is for now a reasonable estimate.
It's still tough out there compared to what the DS/Game Boy/GBA had to compete against .... 3DS and especially Switch have it much harder, even the 3DS was somewhat insulated as smartphone adoption wasn't quite so high (especailly with kids) and tablets were still relatively an expensive item the early years of this decade.
And certainly there weren't like massive multi-million dollar ad campaigns for smartphone games, that would've been unheard of in 2011 as 3DS was launching, today a lot of the most popular games in the industry are mobile games and they have marketing that dwarfs even Playstation/XBox games.
So a lot has changed.
