I think you might be underestimating splat2. It has potential to hit 4mil+. Im going with 5mil for both games. Itll be pretty close imo ;p
4 million+? I don't think so. Splatoon 1 sold about 1.5, on the Wii U granted, but that's still a far cry from 4 and definitely 5 mil. The thing is, I don't think that the Wii U's install base held Splatoon back all that much. To back this up, Mario Kart 8 sold about 1.3 to 7's 2.8 and Smash Wii U sold nearly 0.9 to the 3DS's 2.5. As we can see, MK sold about 47% on Wii U as it did on 3DS and Smash did about 36%. This is in spite of the Wii U only having 14% of the install base.
Now, let's say that the Switch ends with about 18 million LTD in Japan, nearly 6x the Wii U. You may say that this means Spatoon 2 can sell 3x as much, but taking in to account, the Wii U's absurdly high attach-rate and the fact that there is not a direct correlation between install base and game sales, even 3 million, by doubling already massive sales, seems too optimistic.