Pessimist me still thinks because of pricing, lack of true Switch 2 exclusives, and its iterative nature will have sales be more of a slow burn. I have a hard time recommending it to anyone outside of our slice of gaming, especially when a lot of my friends are rocking Steam Decks. But maybe I'm discounting a deep hunger for more MarioKart.
You're once again proving my point about people online living in a bubble. The Steam Deck has sold less than 4 million units since it came out 3 years ago. There's a good chance the Switch 2 either outsold it by the end of last week, or has done so by the end of this week. Plus how many times do I need to say it, Mario Kart 8 Deluxe sold close to 70 million units.
https://www.nintendo.co.jp/ir/en/finance/software/index.html
Mario Kart is literally one of the biggest franchise in gaming. Anyone that wants to play the newest Mario Kart will need to buy a Switch 2. You can't recommend them any other system when no other system will have the newest Mario Kart.
Even when expecting a drop off in sales, when you compare what happened with the DS/Wii era to the 3DS/Wii U era, the best selling Mario Kart of the DS/Wii era was Mario Kart Wii which did 37 million units while the best selling Mario Kart of the 3DS/Wii U era was Mario Kart 7 which did about 19 million units. So even if you had a similar drop between Mario Kart 8 Deluxe and Mario Kart World, Mario Kart World would still end up selling around 35 million copies. Which would literally put it about what Mario Kart Wii sold, which was on a system that did over 100 million units.
Of course keep in mind the main reason for Mario Kart's drop was because of the lose of the Nintendog/Brain Age/Wii Sports/Wii Fit audience that made up a large part of the DS and Wii. The Switch on the other hands biggest sellers behind Mario Kart are Animal Crossing, Smash Bros, 3D Zelda, 3D Mario, Pokemon which all have a history of very constant sales and very dedicated fanbases. Hell in the case of Animal Crossing it did better on the 3DS then it did on the DS, despite the 3DS having only half the audience. Pokemon X/Y only did a million less than Diamond/Pearl, while Sun/Moon did a million better than Black/White.
Nintendo's main franchise from many of it's 20-30 year series, that have had constant if not steady growth over that time span have done better then ever on the Switch. To suddenly expect them to sell even worse then their 3DS counterparts is just silly and would literally require something magical altering reality, or something like a nuclear war creating an apocalypse where nobody can play games anymore. Or course after certain event going on in the world right now, the later is kind of looking somewhat likely. 
You're right, Steam Deck users are still a niche subsection, i'll grant you that.
That said, you're rending the context behind Mario Kart 8 Deluxe's success, a port of a game from the Wii U which hardly anyone owned. The last proper Mario Kart prior to that was Mario Kart 7, a 3DS game from 2011. If you're talking about console Mario Kart, that was Wii back in 2008. If you didn't get a Wii U, that's a 14 year gap from the most recent one and a 17 year gap if you only played on consoles. There's a severe pent-up demand to account for.
I'm not going to prognosticate much because past performance isn't an indicator of future results. I hope it is though.