"It's already the third year for XS and PS5, Nintendo still comes in first place every month"
Switch is dropping because it's past its peak. That doesn't mean it needs to be replaced soon. Sure it dropped 13% and then 20%, and still sold 19.5m last year which was right around PS4's peak year!!! And not much lower than what PS5 and Xbox did combined this year.
If Nintendo keeps bringing the games, does the Nintendo Selects for older games so that a bunch of first party games drop to $30, and does a price cut this summer or next year which could be possibly if supply problem is resolving and parts are getting cheaper again, and maybe even adds a new model like a cheap $200 Switch Home they can definitely keep hardware sales solid while selling mountains of games to their well over 100m userbase Nintendo plans to increase production this year, which I hope would suggest they are planning to do some of these things to spur on sales this year as one would think the only reason to increase production at this point is if they are gonna start making aggressive moves to sell the Switch.
And yes of course consoles need exclusive games, that's what separates them from the other systems. And MK8 was a game on a failed system that hardly anyone owned, while BotW same deal but also launched as the Switch launch game so the Wii U version only sold a little, and they are both Nintendo exclusives, so not exactly good examples of non-exclusive games.
Your last paragraph is confusing because you already said exclusives aren't needed and then you talk about the need for exclusives haha. Sure we should expect next-gen Switch to get XB1 and PS4 multplat ports, as well as likely downgraded ports from XBSeries and PS5. But those can come when the next-gen Nintendo system comes, we don't need them now, there's already a ton to play on Switch without those.
The reality is Nintendo only needs new hardware soon if they choose to need new hardware soon. If they keep the big games coming and give the market price cuts on games and hardware they can easily keep the Switch going into 2025 maybe even to holiday 2025, I certainly wouldn't mind that. However, if they don't make any moves like that and are winding down with the last round of games on Switch then they'll need a next-gen system next year.
We know Zelda ToTK, Pikmin 4, the Wii Kirby remake, and Advance Wars are coming. And I'm still waiting/hoping to see on Switch: 2D Mario, DK, Star Fox, Kid Icarus, 2D Zelda, something in the Excite bike/truck series, Metriod Prime remake, Metroid Prime 4, potentially Prime 2 and 3 in HD, Zelda WW/TP pack, maybe even a Mario Galaxy 2 remaster considering it was very strangely left out of 3D All-Stars. That's a lot of potential first party games that have yet to hit the Switch. There's also possibility of stuff I'd probably not be interested in but others would like another Mario Party, another non-mainline Pokemon game, Mario Baseball, maybe Luigi's Mansion 4. If they have most of this stuff planned for Switch they could easily keep it going until holiday 2025. Or maybe they don't have much more that we don't know about planned for Switch and next-gen will be starting next Spring or something.