Since Valve is determined to keep the Steam Deck as niche as possible, the Switch has literally no competition.
I don't know much about the steam deck, what do you mean by this?
For the last several years, many people in the online gaming bubbles constantly talk about how the Steam Deck has replaced the Switch for them and how the Switch successor will suffer because it won't be the only handheld gaming device anymore. When you look at the actual sales though, the Steam Deck has been out for 3 years now and only sold about 4 million units. In comparison the Wii U which is considered a massive failure that people ignored had sold 12 million units by the time it was 3 years old.
Yeah I know Valve has different expectation for the Steam Deck than Nintendo with the Wii U, but the point still stands that Valve hasn't done anything to make the Steam Deck have greater mainstream appeal. Because of that the Steam Deck is basically a non-factor to the general public and so the Switch 2 has no competition when it comes to a system that allows traditional gaming on a handheld.
This is why the Switch 2 being a Wii U style failure is literally impossible because Nintendo has a monopoly right now on hybrid gaming devices. Anyone that wants to buy the latest Nintendo games and play third party titles on a handheld will need to buy a Switch 2.