Think about it. Going off of VGC numbers, which is roughly accurate enough, attach rates for Splatoon on Wii U are:
NA - 29% (1.64m copies/5.73m consoles)
Europe - 34% (1.02m copies/2.97m consoles)
Japan - 38% (1.17m copies/3.05m consoles)
Elsewhere - 31% (0.25m copies/0.80m consoles)
TOTAL - 33% (4.08m copies/12.55m consoles)
Splatoon is currently on track to become the best selling Wii U game in Japan. It's currently sitting in 3rd place, a mere 20,000 copies behind Mario Kart 8, and 90,000 copies behind New Mario U. Splatoon is becoming a Nintendo juggernaut, especially in Japan.
These numbers are on a console that was doomed before it even launched on store shelves. If Nintendo does everything right with the NX, they will get many, many more people on board. People who didn't play the original Splatoon. If the NX even sells around what the PS4 has now for lifetime, we can whole heartedly expect CoD-like numbers of 10 million units sold for a Splatoon sequel.
Who's with me in this logically not-so bold prediction?
bet: lost
