There's no need for Nintendo to make any efforts to keep the game 'relevant' for the remainder of the year. Splatoon 1 is still being played by enough people to fill up lobbies in a jiffy, and that sold only half the copies of S2 and is on a console that died before the Switch was even released. They already supported the game with much more content that anyone was probably expecting, and it sold more than 8 million copies. If Splatoon 3 doesn't come out for Switch (I'm sure it will, but let's say it doesn't), Splatoon 2 will still be played by enough people by the time the Switch is phased out for the next Nintendo console.
As for Animal Crossing. I don't see how that has anything to do with Splatoon 2. I think Smash and Mario Maker 2 are more direct 'competitors' to Splatoon players' time, as is Fortnite. Animal Crossing is a completely different experience and usually limits the amount of things you can do in a day. Which means that over time, AC daily play time will go down for most players.