Splatoon 2 shipped 10.71m by June 30th. Lifetime sales expectations?

[PAOerfulone]

Somewhere between 17-20 million. But closer to 17 million.

Of all the evergreen titles' legs, Splatoon 2's is probably the weakest, outside of Pokemon Let's Go. It'll keep selling for years and years, but not at the same levels as Breath of the Wild, Super Mario Odyssey, or Smash Bros. Ultimate.

If they ever release a Splatoon 3 on Switch, we all know the 3rd game is coming, we just don't know when, then I could see the renewed interest, hype, and publicity giving a boost to this game's legs. But for now, I don't see anything that suggests Splatoon 3 is currently in the works for the Switch.
So, I'm going with the 17.0-17.9 million option.

Bonus Question: I pay for Nintendo Switch Online. The family plan for $35/year. Myself, two of my best friends, and two of my cousins. They play a lot of online multiplayer between Smash Bros. and Fortnite. Sometimes, I'll do a round of Mario Kart here and there. But I bought it primarily for the NES and SNES stuff.