HyrulianScrolls on 14 July 2017
This seems pretty crazy, I believe there's only 9 games to ever do so, and over half of those are Pokemon games. But if the Switch's install base gets to 15-20m in Japan I suppose it could happen. One thing not being factored in here is that Splatoon 3 releasing for the Switch in 2 years or so shouldn't be ruled out as a possibility, so that could cut into Splatoon 2's lifetime sales a bit.
