Should end up around 15M, Animal Crossing team just launched their game, major updates are probably planned into 2021.
The Splatoon team is probably ramping up work on Splatoon 3 but I don't expect it to be released until 2022/2023.
This quarter thanks to Splatfests and 30% off we saw the game sell more than it did in the holiday quarter. It already had one more such promotion and Splatfest planned for Obon. So by the end of the quarter, it could be around 11.25M, perhaps it will surpass 12M this year with a great holiday performance.

Now one sure way they can get to 15M and surpass it, is to release one or two more updates. There is probably a few PoC they worked on that never made it into the final game. Whether it's Salmon Run, New Mode, New Maps(or old maps from the Wii U debut), new Gear - there is a lot of potential updates they could make while working on Splatoon 3.

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.

Last quarter was suprisingly strong, but aside from that we're looking at a game that's slowing down by quite a bit.
It won't come to a grinding halt or anything, but I'd expect it to land just short of 15 mil unless maybe if Nintendo eventually launches a line of Nintendo Selects for Switch, which Splatoon 2 could be a great candidate for.

Maybe 14-15m?

It will pass 15M, but not go much further than that I think.

This past quarter had a big uptick in sales but I think by next year it'll be back down to low and dwindling quarterly sales. I think it will do another 3 million or so, possible making it to 14 million.

I have Switch Online, it is so cheap there's no point in not getting it even if you only use it every once in a while. I pay for the family plan so my two brothers can have it too, even though I'm pretty sure they almost never use it, maybe a few times a year we play online against one another in Mario Kart or Smash. I don't use it much myself cuz my router doesn't work for online gaming - I can play Tetris 99 but that's about it. I play NES and SNES games occasionally but not that much, but again, at how cheap Switch Online is I'll gladly pay that even just to have the option every once in a while to use it. Splatoon 2 single player is worthless but I've been visiting my bro the past week so I can play online and have been playing Splatoon 2 again for the first time in two years, good times!

btw Nintendo should totally bust out the Nintendo Selects cheaper games for stuff like Splatoon and Let's Go. Splatoon could definitely reach 15 million if they cut the price.

15-17 million is my guess.

As for the bonus question. I do pay for NSO. Use it mainly for Smash online

I dont think we will see Splat3 on Switch. So im guessing 14mil.