If Valve goes through with this and sticks to the device, it could become a serious competitor, though.
But my mom bought a Wii back in the days. For herself. My mom doesn't usually play video games. And she is most certainly not at all interested in playing Doom or Hades (just because they're shown prominently on the webpage). But she liked Wii Fit and bought a Wii.
I honstly think the subset of people who would buy a Steam Deck to avoid buying a switch is so small, it would not put a significant dent in nintendo's bottom line.
The switch is for everyone else - way cheaper, unique games you won't find on a PC, etc. Plus, co-op. I recently spent 6+ hours at my friends house playing Super Mario 3D World Deluxe. They got one joycon, I got the other. In my college dorm, we have a switch dock attached to a monitor in the dorm lounge. Split the joycons, and 4 people can play smash or mario kart. I don't see the deck supporting these use cases.
Valve could try to create games that are more appealing to the typical Nintendo enthusiast, but I suspect the respective cultures are too different.