I also forgot Splatoon 3. That game should also hit 10 million. Specifically, I could see it hitting 11-12 million worldwide (compared to Splatoon 2 probably ending around 16-17 million). I doubt Splatoon 3 will reach 10 million by the end of 2022 though (guessing it will be around 7 million by the end of 2022 though.
Splatoon 2 is not going to sell another 3-4mil after the sequel releases. Even without the sequel it probably wouldn't.