I have neither devices right now, only a PC.
The Steam Deck doesn't cut into Nintendo's core audience, but it does draw away people who would have bought the Switch 1/2 for those reasons -- the audience that made the Switch 1 such an overwhelming success. Anecdotally, I've had multiple non-techies bring up the Steam Deck unprompted, usually with an impression of 'the Switch but better' and/or 'more adult-oriented'.
Historically, when the market they created starts to become saturated, Nintendo starts looking to pivot. So the Steam Deck might not kill the Switch 2, but I'd be very surprised if it doesn't kill the Switch 3.
Steam basically is PC gaming at this point, which is still a massive market that is almost as big as the entirety of console gaming.
I know there are those out there, but I would be slightly surprised if a PC gamer didn't know what a Steam Deck was in 2026.
As someone who has pretty much every console system and most handhelds, I didn't spring for a Switch 2, and it is for the exact reason the thread parent mentions. I do like Nintendo games, as they are consistently high quality, but they are not usually graphics reliant for fun, and the Switch is good enough still, and I don't love paying $90 USD for a single game when I can buy $5-20 games on Steam and play them across multiple devices.
I'm in the same boat as you, I also don't feel the need to buy a Switch 2 yet. Also, game sharing on Steam with my kids is just so much more pleasant. Having multiple kids and multiple Switches was such a shit show of what felt like manual license and provisioning management that I'm not really keen on giving Nintendo more money at this time.
It's not like most people even know what a Steam Deck is. At the moment at least, the two devices mostly don't compete for the same audience. And if you want to play Nintendo's games - which a lot of people do, they're usually quite good - you don't have much choice anyway.