I thought that Splatoon was helping Wii U sell at a fairly steady rate; I hadn't heard about it dropping off this year at all. No doubt, the PS4 will eventually outsell the Wii U in Japan, but the rate at which it's accomplishing this feat isn't exactly rapid.
My bad, it did improve a bit.
1st M: 33 weeks
2nd M: 71 weeks
3rd M: 57 weeks
Although to be fair, the 3rd million covers 2 holiday seasons vs 1 for the previous 2.