Even the next two years, keeping sales 20+ million is not some easy given. That will require Nintendo to expend a lot of heavy hitter software titles to maintain sales and a price cut and likely another model revision on top of that just for that period and even that may not be enough.
When the Wii peaked at 25.9 million after its 2nd full fiscal year on the market, Nintendo did do two price cuts, several new bundles, a revision eventually, and still had multiple big gun games ... a price cut in Sept '09, New Super Mario Bros. Wii, Wii Sports Resort, Wii Fit Plus, Super Mario Galaxy 2, Just Dance breaking out, DKC Returns, Kirby's Epic Yarn, Wii Party, another price cut to $149.99 about 18 months later ... sales still steadily declined every year from that point on by over 5 million units a year for the next couple of years from approx 26m to 20.8m to 15m ... these are not small drops.
The 3DS was still doing pretty OK in fiscal year 3 with about 12.24 million in sales ... you wouldn't expect it to crater in a year to 8.84 million so quickly. Looking at that you would say "well Nintendo probably didn't support the system much, right?", but that's not really right, the 3DS got the New 3DS revision that year plus Super Smash Bros. plus Pokemon Sapphire/Ruby remakes, got Monster Hunter 4 Ultimate, Tomodachi Life, Mario Golf, Kirby Super Deluxe that's not a thin year at all.
Downward momentum can shift against a platform even one that's getting software support, new models, price drops, etc. It happens all the time.