Actually this is a pretty good point. I never noticed the decline was that steady...

As for the software, idk I would guess 5 million for the best seller... MAYBE 10 million if it was a show stopper...

Assuming that Nintendo releases a sucessor for Holiday 2017:

• New Super Mario Bros U - 6 million - Being a launch title and having such a good first year will help this have a HUGE attach rate
• Mario Kart 8 - 5 million - Mario Kart games reliably sell about 1 unit of software for every 3 consoles Nintendo makes a generation
• Super Mario 3D World - 4 million - This would be proportionately similar to Sunshine's sales on the GameCube
• Super Smash Bros 4 - 3.5 million - Fighting games and the Smash phenomenon have declined since 2001, so sales will be the lowest in series history. Nonetheless, a 20% to 25% attach rate would still be likely.
• Donkey Kong Country: Tropical Freeze - 3 million - The 2nd best selling 2D platformer for the system, with half the sales of NSMBU or DKCR.
• Zelda U - 3 million - Somewhere around Wind Waker and Ocarina of Time in terms of proportionate sales.
• Mario Party 10 - 1.5 million - Should sell decently if only because of name and genre.
• Wii Party U - 1.25 million - Will be a long-legged seller
• Animal Crossing 5 - 1 million - Will be decent, thanks to Miiverse and such, but otherwise will disappoint

...of course, this is all hypothetical. 15 million is VERY low.