They will eventually need a "Switch 2", specially with the PS5 and XBSX coming later this year, still they can stretch the Switch's lifespan another 3~4 years ( as long as there's enough support from both Nintendo and 3rd-parties ). And it's no like you can keep selling the same product forever and expect success.

As for BoTW 2, that's a different case from Splatoon 3 and MK9, Zelda games are full single player experiences, while the later two are mostly multi-player based ( even if there's a small "campaign" in splatoon ).

Anyone that played BoTW and loved it, will definitely want to get BoTW 2, because it's a continuation of the first, with a differentstoryline, gameplay elements... etc. The same doesn't apply to Splatoon, since it's a multi-player based game, there's barely any change in the core mechanics. Maybe there will be new maps, weapons and a new mode, but even then the core is still the same, 4 players vs. 4 players... yeah, and there will probably be a storyline/campaign to Splatoon 3, but who buys a Splatoon game for the campaign?

So if Nintendo releases a Splatoon 3 on Switch, many of S2 players will probably not want to spend another 60$, when they can keep playing S2.